diff --git a/src/usexp/lexer.mll b/src/usexp/lexer.mll
index c53bab93..eab1eaf9 100644
--- a/src/usexp/lexer.mll
+++ b/src/usexp/lexer.mll
@@ -76,7 +76,7 @@ let comment   = ';' [^ '\n' '\r']*
 let newline   = '\r'? '\n'
 let blank     = [' ' '\t' '\012']
 let atom_char =
-  [^ ';' '(' ')' '"' ' ' '\t' '\r' '\n' '\000'-'\032' '\127'-'\255']
+  [^ '%' ';' '(' ')' '"' ' ' '\t' '\r' '\n' '\000'-'\032' '\127'-'\255']
 let digit     = ['0'-'9']
 let hexdigit  = ['0'-'9' 'a'-'f' 'A'-'F']
 
diff --git a/src/usexp/usexp.ml b/src/usexp/usexp.ml
index 48a3dd32..684b3751 100644
--- a/src/usexp/usexp.ml
+++ b/src/usexp/usexp.ml
@@ -17,7 +17,7 @@ module Atom = struct
    let rec loop s i len =
      i = len ||
      match String.unsafe_get s i with
-     | '"' | '(' | ')' | ';' | '\000'..'\032' | '\127'..'\255' -> false
+     | '%' | '"' | '(' | ')' | ';' | '\000'..'\032' | '\127'..'\255' -> false
      | _ -> loop s (i + 1) len
    in
    fun s ->