In_category strangeness

Hello. I’m very puzzled by the simple thing.

I have this in my loop:

<?php if (in_category('design')); {
	echo '<b>Design</b>';
}
?>

And surely it echoes “Design” in bold, but is does it in every category (single.php). I also tried:

<?php if ((is_single()) && (in_category('design'))); {
	echo '<b>Design</b>';
}
?>

But with the same result.

What’s going on?

Thanks