new file mode 100644
--- /dev/null
+++ b/compare_locales/integration_tests/__init__.py
@@ -0,0 +1,5 @@
+'''Tests that are not run by default.
+
+They might just take long, or depend on external services, or both.
+They might also fail for external changes.
+'''

new file mode 100644
--- /dev/null
+++ b/compare_locales/integration_tests/test_plurals.py
@@ -0,0 +1,82 @@
+# -*- coding: utf-8 -*-
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+import ast
+import json
+import os
+import unittest
+import urllib2
+
+
+TRANSVISION_URL = (
+    'https://transvision.mozfr.org/'
+    'api/v1/entity/gecko_strings/'
+    '?id=toolkit/chrome/global/intl.properties:pluralRule'
+)
+
+
+class TestPlural(unittest.TestCase):
+    '''Integration test for plural forms and l10n-central.
+
+    Having more plural forms than in l10n-central is OK, missing or
+    mismatching ones isn't.
+    Depends on transvision.
+    '''
+    maxDiff = None
+
+    def test_valid_forms(self):
+        reference_form_map = self._load_transvision()
+        compare_locales_map = self._parse_plurals_py()
+        # Notify locales in compare-locales but not in transvision
+        # Might be incubator locales
+        extra_locales = set()
+        extra_locales.update(compare_locales_map)
+        extra_locales.difference_update(reference_form_map)
+        for locale in sorted(extra_locales):
+            print("{} not in transvision, OK".format(locale))
+            compare_locales_map.pop(locale)
+        # Strip matches from dicts, to make diff for test small
+        locales = set()
+        locales.update(compare_locales_map)
+        locales.intersection_update(reference_form_map)
+        for locale in locales:
+            if compare_locales_map[locale] == reference_form_map[locale]:
+                compare_locales_map.pop(locale)
+                reference_form_map.pop(locale)
+        self.assertDictEqual(reference_form_map, compare_locales_map)
+
+    def _load_transvision(self):
+        '''Use the Transvision API to load all values of pluralRule
+        in intl.properties.
+        Skip test on load failure.
+        '''
+        try:
+            data = urllib2.urlopen(TRANSVISION_URL).read()
+        except urllib2.URLError:
+            raise unittest.SkipTest("Couldn't load transvision API.")
+        return json.loads(data)
+
+    def _parse_plurals_py(self):
+        '''Load compare_locales.plurals, parse the AST, and inspect
+        the dictionary assigned to CATEGORIES_BY_LOCALE to find
+        the actual plural number.
+        Convert both number and locale code to unicode for comparing
+        to json.
+        '''
+        path = os.path.join(os.path.dirname(__file__), '..', 'plurals.py')
+        with open(path) as source_file:
+            plurals_ast = ast.parse(source_file.read())
+        assign_cats_statement = [
+            s for s in plurals_ast.body
+            if isinstance(s, ast.Assign)
+            and any(t.id == 'CATEGORIES_BY_LOCALE' for t in s.targets)
+        ][0]
+        return dict(
+            (unicode(k.s), unicode(v.slice.value.n))
+            for k, v in zip(
+                assign_cats_statement.value.keys,
+                assign_cats_statement.value.values
+            )
+        )

--- a/tox.ini
+++ b/tox.ini
@@ -1,13 +1,15 @@
 [tox]
-envlist = py27, flake8
+envlist = py27, flake8, integration
 
 [travis]
 python =
-  2.7: py27, flake8
+  2.7: py27, flake8, integration
 
 [testenv]
 commands=python setup.py test
 [testenv:flake8]
 deps=flake8 >=3.5, <3.6
 basepython=python2.7
 commands=flake8 compare_locales
+[testenv:integration]
+commands=python -m unittest discover -s compare_locales/integration_tests