Jquery getJSON populate select menu question

$.getJSON('selectMenus.php', function(data){
    var html = '';
    var len = data.length;
    for (var i = 0; i< len; i++) {
        html += '<option value="' + data[i].monthId + '">' + data[i].month + '</option>';
    }
    $('select.month').append(html);
});

Storing the HTML code in a variable and appending it only once at the end is very important if you care about your app performance.

This should work:

   $.getJSON('selectMenus.php', function(data){
        $.each(data, function(index,item) {
           $("select.month").append("<option value=" + item.monthID + ">" + item.month + "</option>"); 
    });
    });

From the great book jQuery in Action,here is a way to do what you want writing a custom jQuery command:

(function($) {
  $.fn.emptySelect = function() {
    return this.each(function(){
      if (this.tagName=='SELECT') this.options.length = 0;
    });
  }

  $.fn.loadSelect = function(optionsDataArray) {
    return this.emptySelect().each(function(){
      if (this.tagName=='SELECT') {
        var selectElement = this;
        $.each(optionsDataArray,function(index,optionData){
          var option = new Option(optionData.caption,
                                  optionData.value);
          if ($.browser.msie) {
            selectElement.add(option);
          }
          else {
            selectElement.add(option,null);
          }
        });
      }
    });
  }
})(jQuery);    

And then:

$.getJSON('selectMenus.php', 
   function(data){
      $("select.month").loadSelect(data);
   }
);

Using @RaYell's method....this is what worked for me:

$.getJSON('months.php', function(data){
    var html = '';
    var len = data.length;
    for (var i = 0; i < len; i++) {html += '<option value="' + data[i].monthId + '">' +    data[i].month + '</option>';
    }
    $('select.month').append(html);
});

Thanks to everyone for your help on this!!