get_posts – exclude one or more categories

Hi folks,

I’ve searched about for answers to this but can’t find anything that works.

How can I modify my query so that it exlucdes a particular category?

<?php $posts = get_posts('numberposts=15'); foreach ($posts as $post) : ?>

I can specify to show only one category, say numberposts=15&category=1, but the reverse, numberposts=15&category=-1 gives me an error.

Confused!

Thanks for your time