diff --git a/test/test_grammars.rb b/test/test_grammars.rb
index 96fdb667..0bbd8923 100644
--- a/test/test_grammars.rb
+++ b/test/test_grammars.rb
@@ -1,8 +1,10 @@
 require_relative "./helper"
 
 class TestGrammars < Test::Unit::TestCase
+  ROOT = File.expand_path("../..", __FILE__)
+
   def setup
-    @grammars = YAML.load(File.read(File.expand_path("../../grammars.yml", __FILE__)))
+    @grammars = YAML.load(File.read(File.join(ROOT, "grammars.yml")))
   end
 
   def test_no_duplicate_scopes
@@ -10,4 +12,26 @@ class TestGrammars < Test::Unit::TestCase
     duplicates = scopes.group_by { |s| s }.select { |k, v| v.length > 1 }.map(&:first)
     assert duplicates.empty?, "The following scopes appear in grammars.yml more than once:\n#{duplicates.sort.join("\n")}"
   end
+
+  def test_submodules_are_in_sync
+    submodules = `git config --list --file "#{File.join(ROOT, ".gitmodules")}"`.lines.grep(/\.path=/).map { |line| line.chomp.split("=", 2).last }
+    # Strip off paths inside the submodule so that just the submodule path remains.
+    listed_submodules = @grammars.keys.grep(/grammar_sources/).map { |source| source[%r{grammar_sources/[^/]+}] }
+
+    nonexistent_submodules = listed_submodules - submodules
+    unlisted_submodules = submodules - listed_submodules
+
+    message = ""
+    unless nonexistent_submodules.empty?
+      message << "The following submodules are listed in grammars.yml but don't seem to exist in the repository. Maybe you should remove them from grammars.yml?\n"
+      message << nonexistent_submodules.sort.join("\n")
+    end
+    unless unlisted_submodules.empty?
+      message << "\n" unless message.empty?
+      message << "The following submodules exist in the repository but aren't listed in grammars.yml. Maybe you should add them to grammars.yml?\n"
+      message << unlisted_submodules.sort.join("\n")
+    end
+
+    assert nonexistent_submodules.empty? && unlisted_submodules.empty?, message
+  end
 end