Using AJAX via POST with multi-select

Using AJAX via POST with multi-select

I have a search form that uses AJAX to fetch data and populate a table.  Several of the search fields are multiple selects.  Problem is, only the first option is getting passed thru to the PHP file called by the AJAX function.
  1. var fs_users = $("#users"), fs_status("#status"), fs_roles("#roles"), ...;
  2. var fs_allFields = $([]).add(fs_users).add(fs_status).add(fs_roles)...;
  3. $("#fsSearch").click(function() {
  4.   //alert(fs_roles.val());
  5.   $.ajax({
  6.     "cache": false,

  7.     "type": "POST",
  8.     "url": "fetch_data.php",
  9.     "data": fs_allFields,
  10.     "dataType": "json",
  11.     "error":...
  12.     "success":...
  13.   });
  14. });
For debugging purposes, the target PHP file ("fetch_data.php") dumps the $_POST and $_GET variables to a file.  As mentioned, only the first option from the multi-selects is getting passed thru in the $_POST variable ($_GET is empty as expected).  The alert (line 4) will correctly display a comma-delimited list of all the selected values (also verified via Firebug while stepping thru the code).  Someone in another thread suggested using serialize() (e.g. line 9 becomes "fs_allFields.serialize()", but that caused NOTHING to get passed thru.

Maybe I've been staring at it too long and there's a simple fix is right in front of me, but I just can't see it.
HELP!

--Mitch

P.S. I'm using the latest versions of jQuery and jQuery-UI.