diff --git a/README.md b/README.md
index 4a53845..61eb287 100644
--- a/README.md
+++ b/README.md
@@ -857,7 +857,7 @@ tach http://example.com
 | `--package-version` / `-p`  | _(none)_                                | Specify an NPM package version to swap in ([details](#swap-npm-dependencies))                                                                                      |
 | `--browser` / `-b`          | `chrome`                                | Which browsers to launch in automatic mode, comma-delimited (chrome, firefox, safari, edge, ie) ([details](#browsers))                                             |
 | `--window-size`             | `1024,768`                              | "width,height" in pixels of the browser windows that will be created                                                                                               |
-| `--sample-size` / `-n`      | `50`                                    | Minimum number of times to run each benchmark ([details](#minimum-sample-size))                                                                                            |
+| `--sample-size` / `-n`      | `50`                                    | Minimum number of times to run each benchmark ([details](#minimum-sample-size))                                                                                    |
 | `--auto-sample-conditions`  | `0%`                                    | The degrees of difference to try and resolve when auto-sampling ("N%" or "Nms", comma-delimited) ([details](#auto-sample-conditions))                              |
 | `--timeout`                 | `3`                                     | The maximum number of minutes to spend auto-sampling ([details](#auto-sample))                                                                                     |
 | `--measure`                 | `callback`                              | Which time interval to measure (`callback`, `global`, `fcp`) ([details](#measurement-modes))                                                                       |
diff --git a/src/cli.ts b/src/cli.ts
index 29ad4e2..67966e8 100644
--- a/src/cli.ts
+++ b/src/cli.ts
@@ -145,7 +145,11 @@ $ tach http://example.com
   for (const {npmInstalls, mountPoints, specs} of plans) {
     promises.push(
       ...npmInstalls.map((install) =>
-        prepareVersionDirectory(install, config.forceCleanNpmInstall)
+        prepareVersionDirectory(
+          install,
+          config.forceCleanNpmInstall,
+          config.npmrc
+        )
       )
     );
     promises.push(
diff --git a/src/config.ts b/src/config.ts
index a24f7de..f1e943e 100644
--- a/src/config.ts
+++ b/src/config.ts
@@ -34,6 +34,7 @@ export interface Config {
   resolveBareModules: boolean;
   remoteAccessibleHost: string;
   forceCleanNpmInstall: boolean;
+  npmrc?: string;
   csvFileStats: string;
   csvFileRaw: string;
 }
@@ -49,6 +50,7 @@ export async function makeConfig(opts: Opts): Promise<Config> {
     csvFileStats: opts['csv-file'],
     csvFileRaw: opts['csv-file-raw'],
     forceCleanNpmInstall: opts['force-clean-npm-install'],
+    npmrc: opts['npmrc'],
     githubCheck: opts['github-check']
       ? parseGithubCheckFlag(opts['github-check'])
       : undefined,
@@ -149,6 +151,7 @@ export function applyDefaults(partial: Partial<Config>): Config {
       partial.forceCleanNpmInstall !== undefined
         ? partial.forceCleanNpmInstall
         : defaults.forceCleanNpmInstall,
+    npmrc: partial.npmrc !== undefined ? partial.npmrc : '',
     githubCheck: partial.githubCheck,
     autoSampleConditions:
       partial.autoSampleConditions !== undefined
diff --git a/src/flags.ts b/src/flags.ts
index b3ac716..57e9dd0 100644
--- a/src/flags.ts
+++ b/src/flags.ts
@@ -92,6 +92,12 @@ export const optDefs: commandLineUsage.OptionDefinition[] = [
     type: Boolean,
     defaultValue: false,
   },
+  {
+    name: 'npmrc',
+    description: `.npmrc file to copy into the test install directory.`,
+    type: String,
+    defaultValue: '',
+  },
   {
     name: 'browser',
     description:
@@ -253,6 +259,7 @@ export interface Opts {
   'remote-accessible-host': string;
   'window-size': string;
   'force-clean-npm-install': boolean;
+  npmrc?: string;
   'csv-file': string;
   'csv-file-raw': string;
   'json-file': string;
diff --git a/src/test/config_test.ts b/src/test/config_test.ts
index b89cb07..b828449 100644
--- a/src/test/config_test.ts
+++ b/src/test/config_test.ts
@@ -33,6 +33,7 @@ suite('makeConfig', function () {
     const argv = ['random-global.html'];
     const expected: Config = {
       mode: 'automatic',
+      npmrc: '',
       sampleSize: 50,
       timeout: 3,
       root: '.',
@@ -77,6 +78,7 @@ suite('makeConfig', function () {
     const argv = ['--config=random-global.json'];
     const expected: Config = {
       mode: 'automatic',
+      npmrc: '',
       sampleSize: 50,
       timeout: 3,
       root: testData,
@@ -122,7 +124,7 @@ suite('makeConfig', function () {
     const argv = ['--config=random-global.json', '--manual'];
     const expected: Config = {
       mode: 'manual',
-
+      npmrc: '',
       sampleSize: 50,
       timeout: 3,
       root: testData,
@@ -173,6 +175,7 @@ suite('makeConfig', function () {
     ];
     const expected: Config = {
       mode: 'automatic',
+      npmrc: '',
       csvFileStats: 'stats.csv',
       csvFileRaw: 'raw.csv',
       jsonFile: 'out.json',
@@ -219,6 +222,7 @@ suite('makeConfig', function () {
     const argv = ['--config=deprecated-horizons.json'];
     const expected: Config = {
       mode: 'automatic',
+      npmrc: '',
       csvFileStats: '',
       csvFileRaw: '',
       jsonFile: '',
diff --git a/src/versions.ts b/src/versions.ts
index 8209f89..5fa1097 100644
--- a/src/versions.ts
+++ b/src/versions.ts
@@ -304,7 +304,8 @@ const installSuccessFile = '__TACHOMETER_INSTALL_SUCCESS__';
  */
 export async function prepareVersionDirectory(
   {installDir, packageJson}: NpmInstall,
-  forceCleanInstall: boolean
+  forceCleanInstall: boolean,
+  npmrc?: string
 ): Promise<void> {
   if (forceCleanInstall) {
     await fsExtra.remove(installDir);
@@ -324,6 +325,13 @@ export async function prepareVersionDirectory(
     path.join(installDir, 'package.json'),
     JSON.stringify(packageJson, null, 2)
   );
+  if (npmrc) {
+    await fsExtra.copy(
+      path.resolve(npmrc),
+      path.join(installDir, '.npmrc'),
+      {}
+    );
+  }
   await runNpm(['install'], {cwd: installDir});
   await fsExtra.writeFile(path.join(installDir, installSuccessFile), '');
 }