diff --git a/src/mzscheme/sconfig.h b/src/mzscheme/sconfig.h
index 2e738b52c8..966658b24f 100644
--- a/src/mzscheme/sconfig.h
+++ b/src/mzscheme/sconfig.h
@@ -110,6 +110,8 @@
 # define USE_ON_EXIT_FOR_ATEXIT
 # endif
 
+#define USE_TRANSITIONAL_64_FILE_OPS
+
 # ifndef i386
 #  define FLUSH_SPARC_REGISTER_WINDOWS
 # endif
@@ -203,6 +205,10 @@
 
 # define MZ_TCP_LISTEN_IPV6_ONLY_SOCKOPT
 
+#if !defined(__x86_64__)
+# define USE_TRANSITIONAL_64_FILE_OPS
+#endif
+
 # define FLAGS_ALREADY_SET
 
 #if defined(i386)
@@ -1023,6 +1029,10 @@
  /* NEED_RESET_STDOUT_BLOCKING enures that file descriptors 1 and 2
     are reset to blocking mode before exiting. */
 
+ /* USE_TRANSITIONAL_64_FILE_OPS uses fseeko64, lseek64, stat64,
+    etc. for file operations involving sizes (that can requires
+    64-bit arithmetic). Use this when off_t is only 32 bits wide. */
+
  /* USE_ULIMIT uses ulimit instead of getdtablesize (Unix). */
 
  /* USE_DYNAMIC_FDSET_SIZE allocates fd_set records based on the
diff --git a/src/mzscheme/src/file.c b/src/mzscheme/src/file.c
index bdcd59b246..ff5bf70cd2 100644
--- a/src/mzscheme/src/file.c
+++ b/src/mzscheme/src/file.c
@@ -5366,7 +5366,7 @@ static Scheme_Object *file_or_dir_permissions(int argc, Scheme_Object *argv[])
 static Scheme_Object *file_size(int argc, Scheme_Object *argv[])
 {
   char *filename;
-  unsigned long len = 0;
+  mzlonglong len = 0;
 
   if (!SCHEME_PATH_STRINGP(argv[0]))
     scheme_wrong_type("file-size", SCHEME_PATH_STRING_STR, 0, argc, argv);
@@ -5378,17 +5378,16 @@ static Scheme_Object *file_size(int argc, Scheme_Object *argv[])
 
 #ifdef DOS_FILE_SYSTEM
  {
-   mzlonglong filesize;
-   if (UNC_stat(filename, strlen(filename), NULL, NULL, NULL, &filesize)) {
-     return scheme_make_integer_value_from_long_long(filesize);
+   if (UNC_stat(filename, strlen(filename), NULL, NULL, NULL, &len)) {
+     return scheme_make_integer_value_from_long_long(len);
    }
  }
 #else
   {
-    struct MSC_IZE(stat) buf;
+    struct MSC_IZE(BIG_OFF_T_IZE(stat)) buf;
 
     while (1) {
-      if (!MSC_W_IZE(stat)(MSC_WIDE_PATH(filename), &buf))
+      if (!MSC_W_IZE(BIG_OFF_T_IZE(stat))(MSC_WIDE_PATH(filename), &buf))
 	break;
       else if (errno != EINTR)
 	goto failed;
@@ -5400,7 +5399,7 @@ static Scheme_Object *file_size(int argc, Scheme_Object *argv[])
     len = buf.st_size;
   }
 
-  return scheme_make_integer_value_from_unsigned(len);
+  return scheme_make_integer_value_from_long_long(len);
 
  failed:
 #endif
diff --git a/src/mzscheme/src/port.c b/src/mzscheme/src/port.c
index d3035d86f9..fda273fc4e 100644
--- a/src/mzscheme/src/port.c
+++ b/src/mzscheme/src/port.c
@@ -87,7 +87,6 @@ extern int osk_not_console; /* set by cmd-line flag */
 
 #define mzAssert(x) /* if (!(x)) abort() */
 
-
 /******************** Generic FILEs ********************/
 
 typedef struct {
@@ -4128,26 +4127,25 @@ scheme_file_position(int argc, Scheme_Object *argv[])
 		     scheme_make_provided_string(argv[0], 2, NULL),
 		     scheme_make_provided_string(argv[1], 2, NULL));
 
-  if ((argc > 1) && SCHEME_BIGNUMP(argv[1]))
-    scheme_raise_exn(MZEXN_FAIL_CONTRACT,
-		     "file-position: new position is too large: %s for port: %s",
-		     scheme_make_provided_string(argv[1], 2, NULL),
-		     scheme_make_provided_string(argv[0], 2, NULL));
-
   if (argc > 1) {
-    long n;
+    mzlonglong nll;
     int whence;
 
-    if (SCHEME_INTP(argv[1])) {
-      n = SCHEME_INT_VAL(argv[1]);
+    if (SCHEME_EOFP(argv[1])) {
+      nll = 0;
+      whence = SEEK_END;
+    } else if (scheme_get_long_long_val(argv[1], &nll)) {
       whence = SEEK_SET;
     } else {
-      n = 0;
-      whence = SEEK_END;
+      scheme_raise_exn(MZEXN_FAIL_CONTRACT,
+                       "file-position: new position is too large: %s for port: %s",
+                       scheme_make_provided_string(argv[1], 2, NULL),
+                       scheme_make_provided_string(argv[0], 2, NULL));
+      return NULL;
     }
       
     if (f) {
-      if (fseek(f, n, whence)) {
+      if (BIG_OFF_T_IZE(fseeko)(f, nll, whence)) {
 	scheme_raise_exn(MZEXN_FAIL_FILESYSTEM,
 			 "file-position: position change failed on file (%e)",
 			 errno);
@@ -4161,20 +4159,14 @@ scheme_file_position(int argc, Scheme_Object *argv[])
       }
       
 # ifdef WINDOWS_FILE_HANDLES
-      lv = SetFilePointer((HANDLE)fd, n, NULL, 
-			  ((whence == SEEK_SET) ? FILE_BEGIN : FILE_END));
-# else
-#  ifdef MAC_FILE_HANDLES
-			  {
-	errno = SetFPos(fd, ((whence == SEEK_SET) ? fsFromStart : fsFromLEOF), n);
-	if (errno == noErr)
-	  lv = 0;
-	else
-	  lv = -1;
+      {
+        
+        if (!SetFilePointerEx((HANDLE)fd, nll, NULL, 
+                              ((whence == SEEK_SET) ? FILE_BEGIN : FILE_END)))
+          lv = -1;
       }
-#  else
-      lv = lseek(fd, n, whence);
-#  endif
+# else
+      lv = BIG_OFF_T_IZE(lseek)(fd, nll, whence);
 # endif
 
       if (lv < 0) {
@@ -4201,6 +4193,16 @@ scheme_file_position(int argc, Scheme_Object *argv[])
       }
 #endif
     } else {
+      long n;
+
+      if (whence == SEEK_SET) {
+        if (!scheme_get_int_val(argv[1], &n)) {
+          scheme_raise_out_of_memory(NULL, NULL);
+        }
+      } else {
+        n = 0;
+      }
+
       if (whence == SEEK_END) {
         if (wis)
           n = is->u.hot;
@@ -4251,64 +4253,59 @@ scheme_file_position(int argc, Scheme_Object *argv[])
 
     return scheme_void;
   } else {
-    long p;
+    mzlonglong pll;
     if (f) {
-      p = ftell(f);
+      pll = BIG_OFF_T_IZE(ftello)(f);
 #ifdef MZ_FDS
     } else if (had_fd) {
 # ifdef WINDOWS_FILE_HANDLES
-      p = SetFilePointer((HANDLE)fd, 0, NULL, FILE_CURRENT);
-# else
-#  ifdef MAC_FILE_HANDLES
       {
-	SInt32 pos;
-	errno = GetFPos(fd, &pos);
-	if (errno == noErr)
-	  p = pos;
-	else
-	  p = -1;
+        LARGE_INTEGER li;
+        if (!SetFilePointerEx((HANDLE)fd, 0, &li, FILE_CURRENT))
+          pll = -1;
+        else
+          pll = li;
       }
-#  else
-      p = lseek(fd, 0, 1);
-#  endif
+# else
+      pll = BIG_OFF_T_IZE(lseek)(fd, 0, 1);
 # endif
-      if (p < 0) {
+      if (pll < 0) {
 	if (SCHEME_INPUT_PORTP(argv[0])) {
-	  p = scheme_tell(argv[0]);
+	  pll = scheme_tell(argv[0]);
 	} else {
-	  p = scheme_output_tell(argv[0]);
+	  pll = scheme_output_tell(argv[0]);
 	}
       } else {
 	if (SCHEME_INPUT_PORTP(argv[0])) {          
           Scheme_Input_Port *ip;
           ip = scheme_input_port_record(argv[0]);
-	  p -= ((Scheme_FD *)ip->port_data)->bufcount;
+	  pll -= ((Scheme_FD *)ip->port_data)->bufcount;
 	} else {
           Scheme_Output_Port *op;
           op = scheme_output_port_record(argv[0]);
-	  p += ((Scheme_FD *)op->port_data)->bufcount;
+	  pll += ((Scheme_FD *)op->port_data)->bufcount;
 	}
       }
 #endif
     } else if (wis)
-      p = is->index;
+      pll = is->index;
     else {
       /* u.pos > index implies we previously moved past the end with file-position */
       if (is->u.pos > is->index)
-	p = is->u.pos;
+	pll = is->u.pos;
       else
-	p = is->index;
+	pll = is->index;
     }
 
     /* Back up for un-gotten & peeked chars: */
     if (SCHEME_INPUT_PORTP(argv[0])) {
       Scheme_Input_Port *ip;
       ip = scheme_input_port_record(argv[0]);
-      p -= ip->ungotten_count;
-      p -= pipe_char_count(ip->peeked_read);
+      pll -= ip->ungotten_count;
+      pll -= pipe_char_count(ip->peeked_read);
     }
 
-    return scheme_make_integer(p);
+    return scheme_make_integer_value_from_long_long(pll);
   }
 }
 
diff --git a/src/mzscheme/src/schpriv.h b/src/mzscheme/src/schpriv.h
index a71f8661ec..f8a8d15b65 100644
--- a/src/mzscheme/src/schpriv.h
+++ b/src/mzscheme/src/schpriv.h
@@ -2565,6 +2565,12 @@ Scheme_Object *scheme_get_native_arity(Scheme_Object *closure);
 /*                         filesystem utilities                           */
 /*========================================================================*/
 
+#ifdef USE_TRANSITIONAL_64_FILE_OPS
+# define BIG_OFF_T_IZE(n) n ## 64
+#else
+# define BIG_OFF_T_IZE(n) n
+#endif
+
 int scheme_is_relative_path(const char *s, long len, int kind);
 int scheme_is_complete_path(const char *s, long len, int kind);