diff --git a/src/c/Makefile.in b/src/c/Makefile.in
index 5aeb36cf7..1bb386312 100644
--- a/src/c/Makefile.in
+++ b/src/c/Makefile.in
@@ -78,8 +78,8 @@ WRITER_OBJS = print.o printer/float_to_digits.o printer/float_to_string.o
 	printer/write_list.o printer/write_code.o printer/write_sse.o           \
 	printer/print_unreadable.o
 
-READER_OBJS = reader.o read.o reader/rtab_cl.o                                  \
-        reader/parse_token.o reader/parse_integer.o reader/parse_number.o
+READER_OBJS = reader.o read.o reader/rtab_cl.o reader/parse_token.o             \
+        reader/parse_fixnum.o reader/parse_integer.o reader/parse_number.o
 
 STREAM_OBJS = stream.o file.o streams/strm_os.o streams/strm_clos.o             \
         streams/strm_string.o streams/strm_composite.o streams/strm_common.o    \
diff --git a/src/c/format.d b/src/c/format.d
index 9dc5aff56..5aa9cb4f3 100644
--- a/src/c/format.d
+++ b/src/c/format.d
@@ -40,19 +40,6 @@ typedef struct format_stack_struct {
   int           nparam;
 } *format_stack;
 
-#if MOST_POSITIVE_FIXNUM_VAL < INT_MAX
-# define FMT_VALUE_UPPER_LIMIT MOST_POSITIVE_FIXNUM
-#else
-# define FMT_VALUE_UPPER_LIMIT INT_MAX
-#endif
-
-#if MOST_NEGATIVE_FIXNUM_VAL > INT_MIN
-# define FMT_VALUE_LOWER_LIMIT MOST_NEGATIVE_FIXNUM
-#else
-# define FMT_VALUE_LOWER_LIMIT INT_MIN
-#endif
-
-
 /******************* COMMON ***************************/
 
 #define NONE    0
@@ -2000,25 +1987,11 @@ format(format_stack fmt, cl_index start, cl_index end)
       do {
         c = ctl_advance(fmt);
       } while (ecl_digitp(c,10) != -1);
-      x = ecl_parse_integer(fmt->ctl_str, i, fmt->ctl_index, &i, 10);
+      x = ecl_parse_fixnum(fmt->ctl_str, i, fmt->ctl_index, &i, 10);
     INTEGER:
-      /* FIXME! A hack to solve the problem of bignums in arguments */
       if (x == OBJNULL || !ecl_numberp(x))
-        fmt_error(fmt, "integer expected");
-      if (ecl_number_compare(x, ecl_make_fixnum(FMT_VALUE_UPPER_LIMIT)) > 0) {
-        fmt->param[n] = ecl_make_fixnum(FMT_VALUE_UPPER_LIMIT);
-      } else if (ecl_number_compare(x, ecl_make_fixnum(FMT_VALUE_LOWER_LIMIT)) < 0) {
-        fmt->param[n] = ecl_make_fixnum(FMT_VALUE_LOWER_LIMIT);
-      } else {
-        fmt->param[n] = x;
-      }
-      if (ECL_FIXNUMP(x)) {
-        fmt->param[n] = x;
-      } else if (ecl_plusp(x)) {
-        fmt->param[n] = ecl_make_fixnum(MOST_POSITIVE_FIXNUM);
-      } else {
-        fmt->param[n] = ecl_make_fixnum(MOST_NEGATIVE_FIXNUM);
-      }
+        fmt_error(fmt, "fixnum expected");
+      fmt->param[n] = x;
       break;
 
     case '\'':
diff --git a/src/c/reader/parse_fixnum.d b/src/c/reader/parse_fixnum.d
new file mode 100644
index 000000000..006617412
--- /dev/null
+++ b/src/c/reader/parse_fixnum.d
@@ -0,0 +1,102 @@
+/* -*- Mode: C; c-basic-offset: 2; indent-tabs-mode: nil -*- */
+/* vim: set filetype=c tabstop=2 shiftwidth=2 expandtab: */
+
+/*
+ *
+ * Copyright (c) 1984 Taiichi Yuasa and Masami Hagiya
+ * Copyright (c) 1990 Giuseppe Attardi
+ * Copyright (c) 2001 Juan Jose Garcia Ripoll
+ * Copyright (c) 2026 Daniel Kochmański
+ *
+ * See file 'LICENSE' for the copyright details.
+ *
+ */
+
+#include <ecl/ecl.h>
+#include <ecl/number.h>
+#include <ecl/internal.h>
+
+#define basep(d) (d <= 36)
+
+static ecl_character
+_ecl_char(cl_object object, cl_index index)
+{
+  switch(ecl_t_of(object)) {
+#ifdef ECL_UNICODE
+  case t_string:
+    return object->string.self[index];
+#endif
+  case t_base_string:
+    return object->base_string.self[index];
+  default:
+    ecl_internal_error("ecl_parse_fixnum: source is not a string.");
+  }
+}
+
+static int
+_ecl_digitp(ecl_character i, int r)
+{
+  if (('0' <= i) && (i <= '9') && (i < '0' + r))
+    return i - '0';
+  if (('A' <= i) && (10 < r) && (i < 'A' + (r - 10)))
+    return i - 'A' + 10;
+  if (('a' <= i) && (10 < r) && (i < 'a' + (r - 10)))
+    return i - 'a' + 10;
+/* #ifdef ECL_UNICODE */
+/*   if (i > 255) { */
+/*     int number = ucd_decimal_digit(i); */
+/*     if (number < r) */
+/*       return number; */
+/*   } */
+/* #endif */
+  return -1;
+}
+
+/* This is like ecl_parse_integer (including the API), but returns OBJNULL also
+   when the result does not fit in a fixnum. */
+cl_object
+ecl_parse_fixnum(cl_object str, cl_index start, cl_index end,
+                 cl_index *ep, unsigned int radix)
+{
+  int sign, d;
+  cl_fixnum aux, integer_part;
+  cl_object output;
+  cl_index i, c;
+
+  if (start >= end || !basep(radix)) {
+    *ep = start;
+    return OBJNULL;
+  }
+  sign = 1;
+  c = _ecl_char(str, start);
+  if (c == '+') {
+    start++;
+  } else if (c == '-') {
+    sign = -1;
+    start++;
+  }
+  integer_part = 0;
+  for (i = start; i < end; i++) {
+    c = _ecl_char(str, i);
+    d = _ecl_digitp(c, radix);
+    if (d < 0) {
+      break;
+    }
+    aux = integer_part;
+    integer_part *= radix;
+    if (integer_part/radix != aux) {
+      *ep = i-1;
+      return OBJNULL;
+    }
+    integer_part += d;
+    if (integer_part > MOST_POSITIVE_FIXNUM) {
+      *ep = i-1;
+      return OBJNULL;;
+    }
+  }
+  if (sign < 0) {
+    integer_part = -integer_part;
+  }
+  *ep = i;
+  return (i == start)? OBJNULL : ecl_make_fixnum(integer_part);
+}
diff --git a/src/h/external.h b/src/h/external.h
index b8a791c95..1f292107e 100755
--- a/src/h/external.h
+++ b/src/h/external.h
@@ -1582,6 +1582,7 @@ extern ECL_API cl_object ecl_read_object(cl_object in);
 extern ECL_API cl_object ecl_read_token(cl_object rtbl, cl_object in, int flags);
 extern ECL_API cl_object ecl_parse_token(cl_object token, cl_object in, int flags);
 extern ECL_API cl_object ecl_parse_number(cl_object s, cl_index start, cl_index end, cl_index *ep, unsigned int radix);
+extern ECL_API cl_object ecl_parse_fixnum(cl_object s, cl_index start, cl_index end, cl_index *ep, unsigned int radix);
 extern ECL_API cl_object ecl_parse_integer(cl_object s, cl_index start, cl_index end, cl_index *ep, unsigned int radix);
 extern ECL_API bool ecl_invalid_character_p(int c);
 extern ECL_API cl_object ecl_copy_readtable(cl_object from, cl_object to);