select varchar fields in table1 that are not in table2

CREATE TABLE `saved_searches` (
  `savedSearchesID` bigint(20) unsigned NOT NULL auto_increment,
  `savedSearchesDefaultUsername` varchar(50) default NULL,
  PRIMARY KEY  (`savedSearchesID`)
);

 CREATE TABLE `a_messages` (
  `a_messages_id` int(11) NOT NULL auto_increment,
  `profile_id` varchar(20) default NULL,
  `message_id` bigint(20) default NULL,
  `this_user` varchar(20) default NULL,
  PRIMARY KEY  (`a_messages_id`),
  UNIQUE KEY `unique_message_id` (`message_id`)
) ENGINE=MyISAM AUTO_INCREMENT=51 DEFAULT CHARSET=utf8;

saved_searches.savedSearchesDefaultUsername is the same as a_messages.profile_id
want to select all the
saved_searches.savedSearchesDefaultUsername
that are not yet in
a_messages.profile_id

sample data
saved_searches.savedSearchesDefaultUsername
bob
sarah
susan

 a_messages.profile_id
bob

so only select
sarah and susan