diff --git a/include/netcdf.h b/include/netcdf.h
index f0863f570..3a7bf6e33 100644
--- a/include/netcdf.h
+++ b/include/netcdf.h
@@ -158,8 +158,10 @@ Use this in mode flags for both nc_create() and nc_open(). */
 
 #define NC_PNETCDF       (NC_MPIIO) /**< Use parallel-netcdf library; alias for NC_MPIIO. */
 
-#define NC_UDF0          0x0080  /**< User-defined format. */
-#define NC_UDF1          0x0002  /**< User-defined format. */
+#define NC_UDF0          0x0080  /**< User-defined format 0. */
+#define NC_UDF1          0x0002  /**< User-defined format 1. */
+
+#define NC_MAX_MAGIC_NUMBER_LEN 8 /**< Max len of ser-defined format magic number. */
 
 /** Format specifier for nc_set_default_format() and returned
  *  by nc_inq_format. This returns the format as provided by
diff --git a/libdispatch/dfile.c b/libdispatch/dfile.c
index ba4b447c5..c02a2331d 100644
--- a/libdispatch/dfile.c
+++ b/libdispatch/dfile.c
@@ -67,7 +67,9 @@ static char HDF5_SIGNATURE[MAGIC_NUMBER_LEN] = "\211HDF\r\n\032\n";
 #ifdef USE_NETCDF4
 /* User-defined formats. */
 NC_Dispatch *UDF0_dispatch_table = NULL;
+char UDF0_magic_number[NC_MAX_MAGIC_NUMBER_LEN + 1];
 NC_Dispatch *UDF1_dispatch_table = NULL;
+char UDF1_magic_number[NC_MAX_MAGIC_NUMBER_LEN + 1];
 #endif /* USE_NETCDF4 */
 
 /** \defgroup datasets NetCDF File and Data I/O
@@ -128,15 +130,22 @@ nc_def_user_format(int mode_flag, NC_Dispatch *dispatch_table, char *magic_numbe
       return NC_EINVAL;
    if (!dispatch_table)
       return NC_EINVAL;
+   if (magic_number && strlen(magic_number) > NC_MAX_MAGIC_NUMBER_LEN)
+      return NC_EINVAL;      
 
-   /* Retain a pointer to the dispatch_table. */
+   /* Retain a pointer to the dispatch_table and a copy of the magic
+    * number, if one was provided. */
    switch(mode_flag)
    {
    case NC_UDF0:
       UDF0_dispatch_table = dispatch_table;
+      if (magic_number)
+         strncpy(UDF0_magic_number, magic_number, NC_MAX_MAGIC_NUMBER_LEN);
       break;
    case NC_UDF1:
       UDF1_dispatch_table = dispatch_table;
+      if (magic_number)
+         strncpy(UDF0_magic_number, magic_number, NC_MAX_MAGIC_NUMBER_LEN);
       break;
    }
    
diff --git a/nc_test4/tst_udf.c b/nc_test4/tst_udf.c
index 4ff0f262c..d849f977b 100644
--- a/nc_test4/tst_udf.c
+++ b/nc_test4/tst_udf.c
@@ -191,12 +191,14 @@ main(int argc, char **argv)
    printf("*** testing user-defined format with magic number...");
    {
       int ncid;
+      char magic_number[5] = "1111";
+      
       /* Create an empty file to play with. */
       if (nc_create(FILE_NAME, 0, &ncid)) ERR;
       if (nc_close(ncid)) ERR;
       
       /* Add our test user defined format. */
-      if (nc_def_user_format(NC_UDF0, &tst_dispatcher, NULL)) ERR;
+      if (nc_def_user_format(NC_UDF0, &tst_dispatcher, magic_number)) ERR;
 
       /* Open file with our defined functions. */
       if (nc_open(FILE_NAME, NC_UDF0, &ncid)) ERR;