diff --git a/Library/Formula/ctags.rb b/Library/Formula/ctags.rb
index 40c0533d3f6c..89ce77862e34 100644
--- a/Library/Formula/ctags.rb
+++ b/Library/Formula/ctags.rb
@@ -7,6 +7,9 @@ class Ctags < Formula
 
   head 'https://ctags.svn.sourceforge.net/svnroot/ctags/trunk'
 
+  conflicts_with 'emacs',
+    :because => 'Both ctags and emacs install a `ctags` executable.'
+
   depends_on :autoconf => :build if build.head?
 
   def patches
diff --git a/Library/Formula/emacs.rb b/Library/Formula/emacs.rb
index f7c8406674de..bdd5be9fdbf4 100644
--- a/Library/Formula/emacs.rb
+++ b/Library/Formula/emacs.rb
@@ -6,6 +6,9 @@ class Emacs < Formula
   mirror 'http://ftp.gnu.org/pub/gnu/emacs/emacs-24.2.tar.bz2'
   sha1 '38e8fbc9573b70a123358b155cf55c274b5a56cf'
 
+  conflicts_with 'ctags',
+    :because => 'Both emacs and ctags install a `ctags` executable.'
+
   option "cocoa", "Build a Cocoa version of emacs"
   option "srgb", "Enable sRGB colors in the Cocoa version of emacs"
   option "with-x", "Include X11 support"