<?php
        if (is_category()) {
        $this_category = get_category($cat);
        }
        ?>


        <?php
        if($this_category->category_parent)
        $this_category = wp_list_categories('orderby=id&show_count=0&title_li=&use_desc_for_title=1&child_of='.$this_category->category_parent."&echo=0");
        else
        $this_category = wp_list_categories('orderby=id&depth=1&show_count=0&title_li=&use_desc_for_title=1&child_of='.$this_category->cat_ID."&echo=0");
        if ($this_category) { ?>

        <ul>
        <?php echo $this_category; ?>
        </ul>

        <?php } ?>