diff --git a/lib/linguist/heuristics.rb b/lib/linguist/heuristics.rb
index 9c1d9948..13f0173f 100644
--- a/lib/linguist/heuristics.rb
+++ b/lib/linguist/heuristics.rb
@@ -36,6 +36,9 @@ module Linguist
         if languages.all? { |l| ["AsciiDoc", "AGS Script"].include?(l) }
           result = disambiguate_asc(data)
         end
+        if languages.all? { |l| ["FORTRAN", "Forth"].include?(l) }
+          result = disambiguate_f(data)
+        end
         return result
       end
     end
@@ -138,6 +141,16 @@ module Linguist
       matches
     end
 
+    def self.disambiguate_f(data)
+      matches = []
+      if /^: /.match(data)
+        matches << Language["Forth"]
+      elsif /^([c*][^a-z]|      subroutine\s)/i.match(data)
+        matches << Language["FORTRAN"]
+      end
+      matches
+    end
+
     def self.active?
       !!ACTIVE
     end
diff --git a/lib/linguist/languages.yml b/lib/linguist/languages.yml
index 022918e6..ac284ca4 100644
--- a/lib/linguist/languages.yml
+++ b/lib/linguist/languages.yml
@@ -792,6 +792,9 @@ Forth:
   extensions:
   - .fth
   - .4th
+  - .F
+  - .f
+  - .for
   - .forth
   - .frt
 
diff --git a/test/test_heuristics.rb b/test/test_heuristics.rb
index 3e04daf3..db8f286e 100644
--- a/test/test_heuristics.rb
+++ b/test/test_heuristics.rb
@@ -105,6 +105,16 @@ class TestHeuristcs < Test::Unit::TestCase
     end
   end
 
+  def test_f_by_heuristics
+    languages = ["FORTRAN", "Forth"]
+    languages.each do |language|
+      all_fixtures(language).each do |fixture|
+        results = Heuristics.disambiguate_f(fixture("#{language}/#{File.basename(fixture)}"))
+        assert_equal Language[language], results.first
+      end
+    end
+  end
+
   # Candidate languages = ["Hack", "PHP"]
   def test_hack_by_heuristics
     results = Heuristics.disambiguate_hack(fixture("Hack/funs.php"))