In your single.php just replace this line:

<?php if (have_posts()) : ?>

With this one:

<?php
    if ( have_posts() ) { the_post(); rewind_posts(); }
        if ( in_category(59) ) {
	    include(TEMPLATEPATH . '/singlepress.php');
        }
?>

Just replace “singlepress.php” with your actual template filename and the category ID with the actual category ID