diff --git a/lib/linguist/heuristics.rb b/lib/linguist/heuristics.rb
index 97c1015a..33df8958 100644
--- a/lib/linguist/heuristics.rb
+++ b/lib/linguist/heuristics.rb
@@ -69,6 +69,14 @@ module Linguist
       end
     end
 
+    disambiguate "C#", "Smalltalk" do |data|
+      if /![\w\s]+methodsFor: /.match(data)
+        Language["Smalltalk"]
+      elsif /^\s*namespace\s*[\w\.]+\s*{/.match(data) || /^\s*using\s*[\w\.]+\s*;/.match(data)
+        Language["C#"]
+      end
+    end
+
     disambiguate "Objective-C", "C++", "C" do |data|
       if (/^[ \t]*@(interface|class|protocol|property|end|synchronised|selector|implementation)\b/.match(data))
         Language["Objective-C"]
diff --git a/test/test_heuristics.rb b/test/test_heuristics.rb
index 7520e828..785fe58e 100644
--- a/test/test_heuristics.rb
+++ b/test/test_heuristics.rb
@@ -133,6 +133,13 @@ class TestHeuristcs < Minitest::Test
     })
   end
 
+  def test_cs_by_heuristics
+    assert_heuristics({
+      "C#" => all_fixtures("C#", "*.cs"),
+      "Smalltalk" => all_fixtures("Smalltalk", "*.cs")
+    })
+  end
+
   def assert_heuristics(hash)
     candidates = hash.keys.map { |l| Language[l] }