diff --git a/syntax/quox.vim b/syntax/quox.vim
index 158d7d9..33ff89f 100644
--- a/syntax/quox.vim
+++ b/syntax/quox.vim
@@ -32,6 +32,7 @@ hi def link quoxBuiltinVal Function
 
 syn match quoxNat /\<\d\%(\%(\d\|_\)*\d\)\?\>/
 syn match quoxNat /\c\<0x\x\%(\%(\x\|_\)*\x\)\?\>/
+syn keyword quoxNat zero succ
 hi def link quoxNat Number
 
 syn region quoxString start=+"+ end=+"+ skip=+\\"+ contains=quoxCharEsc