diff --git a/ars.lua b/ars.lua
index cc9431a..99e3c46 100644
--- a/ars.lua
+++ b/ars.lua
@@ -201,7 +201,7 @@ function Ars:tell(scored, unperturbed_score)
         self._params[i] = v + self.learning_rate * step[i]
     end
 
-    self.asked = nil
+    self.noise = nil
 end
 
 return {
diff --git a/config.lua b/config.lua
index a3b00ba..3106c45 100644
--- a/config.lua
+++ b/config.lua
@@ -90,6 +90,8 @@ assert(not cfg.ars_lips or cfg.unperturbed_trial,
        "cfg.unperturbed_trial must be true to use cfg.ars_lips")
 assert(not cfg.ars_lips or cfg.negate_trials,
        "cfg.negate_trials must be true to use cfg.ars_lips")
+assert(not cfg.es == 'xnes' or not cfg.negate_trials,
+       "cfg.negate_trials is not yet compatible with xNES")
 
 assert(not cfg.adamant,
        "cfg.adamant not yet re-implemented")
diff --git a/main.lua b/main.lua
index e51e169..0b92d01 100644
--- a/main.lua
+++ b/main.lua
@@ -50,27 +50,28 @@ local last_trial_state
 
 -- localize some stuff.
 
-local assert = assert
-local print = print
-local ipairs = ipairs
-local pairs = pairs
-local select = select
-local open = io.open
 local abs = math.abs
-local floor = math.floor
+local assert = assert
 local ceil = math.ceil
-local min = math.min
-local max = math.max
+local collectgarbage = collectgarbage
 local exp = math.exp
-local pow = math.pow
+local floor = math.floor
+local insert = table.insert
+local ipairs = ipairs
 local log = math.log
-local sqrt = math.sqrt
+local max = math.max
+local min = math.min
+local open = io.open
+local pairs = pairs
+local pow = math.pow
+local print = print
 local random = math.random
 local randomseed = math.randomseed
-local insert = table.insert
 local remove = table.remove
-local unpack = table.unpack or unpack
+local select = select
 local sort = table.sort
+local sqrt = math.sqrt
+local unpack = table.unpack or unpack
 
 local band = bit.band
 local bor = bit.bor
@@ -173,6 +174,7 @@ end
 -- learning and evaluation.
 
 local ars = require("ars")
+local xnes = require("xnes")
 
 local function prepare_epoch()
     trial_neg = false
@@ -180,7 +182,7 @@ local function prepare_epoch()
     base_params = network:collect()
     if cfg.playback_mode then return end
 
-    print('preparing epoch '..tostring(epoch_i)..'.')
+    print('preparing epoch '..tostring(epoch_i)..'...')
     empty(trial_rewards)
 
     local precision
@@ -190,7 +192,7 @@ local function prepare_epoch()
     end
 
     local dummy
-    if es == 'ars' then
+    if cfg.es == 'ars' then
         trial_params, dummy = es:ask(precision)
     else
         trial_params, dummy = es:ask()
@@ -236,12 +238,16 @@ local function learn_from_epoch()
         end
     end
 
-    if es == 'ars' then
+    if cfg.es == 'ars' and cfg.ars_lips then
         es:tell(trial_rewards, current_cost)
     else
         es:tell(trial_rewards)
     end
 
+    if cfg.es == 'xnes' then
+        print("sigma:", es.sigma)
+    end
+
     local step_mean, step_dev = 0, 0
     --[[ TODO
     local step_mean, step_dev = calc_mean_dev(step)
@@ -369,6 +375,7 @@ local function do_reset()
         if epoch_i > 0 then learn_from_epoch() end
         if not cfg.playback_mode then epoch_i = epoch_i + 1 end
         prepare_epoch()
+        collectgarbage()
         if any_random then
             loadlevel(cfg.starting_world, cfg.starting_level)
             state_saved = false
@@ -438,7 +445,14 @@ local function init()
     local res, err = pcall(network.load, network, cfg.params_fn)
     if res == false then print(err) end
 
-    if cfg.es == 'ars' then
+    if cfg.es == 'xnes' then
+        -- if you get an out of memory error, you can't use xNES. sorry!
+        -- maybe there'll be a patch for FCEUX in the future.
+        local trials = cfg.epoch_trials
+        if cfg.negate_trials then trials = trials * 2 end
+        es = xnes.Xnes(network.n_param, trials, cfg.learning_rate,
+                       cfg.deviation, cfg.negate_trials)
+    elseif cfg.es == 'ars' then
         es = ars.Ars(network.n_param, cfg.epoch_trials, cfg.epoch_top_trials,
                      cfg.learning_rate, cfg.deviation, cfg.negate_trials)
     else
diff --git a/xnes.lua b/xnes.lua
new file mode 100644
index 0000000..b3c2fe5
--- /dev/null
+++ b/xnes.lua
@@ -0,0 +1,206 @@
+-- Exponential Natural Evolution Strategies
+-- http://people.idsia.ch/~juergen/xNES2010gecco.pdf
+-- not to be confused with the Nintendo Entertainment System.
+
+local assert = assert
+local exp = math.exp
+local floor = math.floor
+local ipairs = ipairs
+local log = math.log
+local max = math.max
+local pairs = pairs
+local pow = math.pow
+local sqrt = math.sqrt
+local unpack = table.unpack or unpack
+
+local Base = require "Base"
+
+local nn = require "nn"
+local normal = nn.normal
+local zeros = nn.zeros
+
+local util = require "util"
+local argsort = util.argsort
+
+local Xnes = Base:extend()
+
+local function dot_mv(mat, vec, out)
+    -- treats matrix as a matrix.
+    -- treats vec as a column vector, flattened.
+    assert(#mat.shape == 2)
+    local d0, d1 = unpack(mat.shape)
+    assert(d1 == #vec)
+
+    local out_shape = {d0}
+    if out == nil then
+        out = zeros(out_shape)
+    else
+        assert(d0 == #out, "given output is the wrong size")
+    end
+
+    for i=1, d0 do
+        local sum = 0
+        for j=1, d1 do
+            sum = sum + mat[(i - 1) * d1 + j] * vec[j]
+        end
+        out[i] = sum
+    end
+
+    return out
+end
+
+local function make_utility(popsize, out)
+    local utility = out or {}
+    local temp = log(popsize / 2 + 1)
+    for i=1, popsize do utility[i] = max(0, temp - log(i)) end
+    local sum = 0
+    for _, v in ipairs(utility) do sum = sum + v end
+    for i, v in ipairs(utility) do utility[i] = v / sum - 1 / popsize end
+    return utility
+end
+
+local function make_covars(dims, sigma, out)
+    local covars = out or zeros{dims, dims}
+    local c = sigma / dims
+    -- simplified form of the determinant of the matrix we're going to create:
+    local det = pow(1 - c, dims - 1) * (c * (dims - 1) + 1)
+    -- multiplying by this constant makes the determinant 1:
+    local m = pow(1 / det, 1 / dims)
+
+    local filler = c * m
+    for i=1, #covars do covars[i] = filler end
+    -- diagonals:
+    for i=1, dims do covars[i + dims * (i - 1)] = m end
+
+    return covars
+end
+
+function Xnes:init(dims, popsize, learning_rate, sigma)
+    -- heuristic borrowed from CMA-ES:
+    self.dims = dims
+    self.popsize = popsize or 4 + (3 * floor(log(dims)))
+    self.learning_rate = learning_rate or 3/5 * (3 + log(dims)) / (dims * sqrt(dims))
+    self.sigma = sigma or 1
+
+    self.utility = make_utility(self.popsize)
+
+    self.mean = zeros{dims}
+    -- note: this is technically the co-standard-deviation.
+    -- you can imagine the "s" standing for "sqrt" if you like.
+    self.covars = make_covars(self.dims, self.sigma, self.covars)
+
+    --self.log_sigma = log(self.sigma)
+    --self.log_covars = zeros{dims, dims}
+    --for i, v in ipairs(self.covars) do self.log_covars[i] = log(v) end
+end
+
+function Xnes:params(new_mean, new_covars)
+    if new_mean ~= nil then
+        assert(#self.mean == #new_mean, "new parameters have the wrong size")
+        for i, v in ipairs(new_mean) do self.mean[i] = v end
+    end
+    if new_covars ~= nil then
+        -- TODO: assert determinant of new_covars is 1.
+        error("TODO")
+    end
+    return self.mean
+end
+
+function Xnes:ask_once(asked, noise)
+    asked = asked or zeros(self.dims)
+    noise = noise or {}
+
+    for i=1, self.dims do noise[i] = normal() end
+    noise.shape = {#noise}
+
+    dot_mv(self.covars, noise, asked)
+    for i, v in ipairs(asked) do asked[i] = self.mean[i] + self.sigma * v end
+
+    return asked, noise
+end
+
+function Xnes:ask(asked, noise)
+    -- return a list of parameters for the user to score,
+    -- and later pass to :tell().
+    if asked == nil then
+        asked = {}
+        for i=1, self.popsize do asked[i] = zeros(self.dims) end
+    end
+    if noise == nil then
+        noise = {}
+        for i=1, self.popsize do noise[i] = zeros(self.dims) end
+    end
+    for i=1, self.popsize do self:ask_once(asked[i], noise[i]) end
+    self.noise = noise
+    return asked, noise
+end
+
+function Xnes:tell(scored, noise)
+    local noise = noise or self.noise
+    assert(noise, "missing noise argument")
+
+    local arg = argsort(scored, function(a, b) return a > b end)
+
+    local g_delta = zeros{self.dims}
+    for p=1, self.popsize do
+        local noise_p = noise[arg[p]]
+        for i=1, self.dims do
+            g_delta[i] = g_delta[i] + self.utility[p] * noise_p[i]
+        end
+    end
+
+    local g_covars = zeros{self.dims, self.dims}
+    local traced = 0
+    for p=1, self.popsize do
+        local noise_p = noise[arg[p]]
+        for i=1, self.dims do
+            for j=1, self.dims do
+                local ind = (i - 1) * self.dims + j
+                local zzt = noise_p[i] * noise_p[j] - (i == j and 1 or 0)
+                local temp = self.utility[p] * zzt
+                g_covars[ind] = g_covars[ind] + temp
+                traced = traced + temp
+            end
+        end
+    end
+
+    local g_sigma = traced / self.dims
+
+    for i=1, self.dims do
+        local ind = (i - 1) * self.dims + i
+        g_covars[ind] = g_covars[ind] - g_sigma
+    end
+
+    -- finally, update according to the gradients.
+
+    local dotted = dot_mv(self.covars, g_delta)
+    for i, v in ipairs(self.mean) do
+        self.mean[i] = v + self.sigma * dotted[i]
+    end
+
+    --[[
+    --self.log_sigma = self.log_sigma + self.learning_rate / 2 * g_sigma
+    for i, v in ipairs(self.log_covars) do
+        self.log_covars[i] = v + lr * g_covars[i]
+    end
+    --]]
+
+    local lr = self.learning_rate * 0.5
+    self.sigma = self.sigma * exp(lr * g_sigma)
+    for i, v in ipairs(self.covars) do
+        self.covars[i] = v * exp(lr * g_covars[i])
+    end
+
+    -- bookkeeping:
+    --self.sigma = exp(self.log_sigma)
+    --for i, v in ipairs(self.log_covars) do self.covars[i] = exp(v) end
+    self.noise = nil
+end
+
+return {
+    dot_mv = dot_mv,
+    make_utility = make_utility,
+    make_covars = make_covars,
+
+    Xnes = Xnes,
+}