diff --git a/byterun/floats.c b/byterun/floats.c
index c8684f9ad..c1f6ec5af 100644
--- a/byterun/floats.c
+++ b/byterun/floats.c
@@ -104,10 +104,23 @@ CAMLprim value format_float(value fmt, value arg)
 
 CAMLprim value float_of_string(value vs)
 {
-  char * s = String_val(vs);
-  char * ends;
-  double d = strtod((const char *) s, &ends);
-  if (ends != s + string_length(vs)) failwith("float_of_string");
+  char parse_buffer[64];
+  char * buf, * src, * dst, * end;
+  mlsize_t len;
+  double d;
+
+  len = string_length(vs);
+  buf = len < sizeof(parse_buffer) ? parse_buffer : stat_alloc(len + 1);
+  src = String_val(vs);
+  dst = buf;
+  while (len--) {
+    char c = *src++;
+    if (c != '_') *dst++ = c;
+  }
+  *dst = 0;
+  d = strtod((const char *) buf, &end);
+  if (buf != parse_buffer) stat_free(buf);
+  if (end != dst) failwith("float_of_string");
   return copy_double(d);
 }
 
diff --git a/byterun/ints.c b/byterun/ints.c
index 9dbf2b373..e9ad19401 100644
--- a/byterun/ints.c
+++ b/byterun/ints.c
@@ -46,9 +46,8 @@ static char * parse_sign_and_base(char * p,
   return p;
 }
 
-static int parse_digit(char * p)
+static int parse_digit(char c)
 {
-  int c = *p;
   if (c >= '0' && c <= '9')
     return c - '0';
   else if (c >= 'A' && c <= 'F')
@@ -65,10 +64,12 @@ static long parse_long(char * p)
   int sign, base, d;
 
   p = parse_sign_and_base(p, &base, &sign);
-  d = parse_digit(p);
+  d = parse_digit(*p);
   if (d < 0 || d >= base) failwith("int_of_string");
   for (p++, res = d; /*nothing*/; p++) {
-    d = parse_digit(p);
+    char c = *p;
+    if (c == '_') continue;
+    d = parse_digit(c);
     if (d < 0 || d >= base) break;
     res = base * res + d;
   }
@@ -421,10 +422,12 @@ CAMLprim value int64_of_string(value s)
   int sign, base, d;
 
   p = parse_sign_and_base(String_val(s), &base, &sign);
-  d = parse_digit(p);
+  d = parse_digit(*p);
   if (d < 0 || d >= base) failwith("int_of_string");
   for (p++, res = d; /*nothing*/; p++) {
-    d = parse_digit(p);
+    char c = *p;
+    if (c == '_') continue;
+    d = parse_digit(c);
     if (d < 0 || d >= base) break;
     res = base * res + d;
   }
diff --git a/parsing/lexer.mll b/parsing/lexer.mll
index 8e71d1a72..4701ee294 100644
--- a/parsing/lexer.mll
+++ b/parsing/lexer.mll
@@ -180,12 +180,18 @@ let identchar =
   ['A'-'Z' 'a'-'z' '_' '\192'-'\214' '\216'-'\246' '\248'-'\255' '\'' '0'-'9']
 let symbolchar =
   ['!' '$' '%' '&' '*' '+' '-' '.' '/' ':' '<' '=' '>' '?' '@' '^' '|' '~']
-let decimal_literal = ['0'-'9']+
-let hex_literal = '0' ['x' 'X'] ['0'-'9' 'A'-'F' 'a'-'f']+
-let oct_literal = '0' ['o' 'O'] ['0'-'7']+
-let bin_literal = '0' ['b' 'B'] ['0'-'1']+
+let decimal_literal =
+  ['0'-'9'] ['0'-'9' '_']*
+let hex_literal =
+  '0' ['x' 'X'] ['0'-'9' 'A'-'F' 'a'-'f']['0'-'9' 'A'-'F' 'a'-'f' '_']*
+let oct_literal =
+  '0' ['o' 'O'] ['0'-'7'] ['0'-'7' '_']*
+let bin_literal =
+  '0' ['b' 'B'] ['0'-'1'] ['0'-'1']*
 let float_literal =
-  ['0'-'9']+ ('.' ['0'-'9']* )? (['e' 'E'] ['+' '-']? ['0'-'9']+)?
+  ['0'-'9'] ['0'-'9' '_']* 
+  ('.' ['0'-'9' '_']* )?
+  (['e' 'E'] ['+' '-']? ['0'-'9']+)?
 
 rule token = parse
     blank +