Duplicate attribute class causing site validation error

The following bit of code results in a duplicate attribute site validation error.

This template script:

<div class="clearfix" <?php post_class() ?>>

produces this source output:

<div class="clearfix" class="post-263 post type-post status-publish format-st…

WordPress adds its own post-related classes to format the blog entry. Is there any way I can add my “clearfix” class to the template without producing the duplicate attributes?

Thanks.