Transparent background?

Ok, hope someone knows if this is possible and how to go about doing it. I’ve got a flash movie that will be centered in the browser. It is only about 700 px wide and 400 heigh. Now, in my html I have tiled background. In my flash movie there are seperate boxes which display the information and so forth. I wan’t the html background to show through the movie so it fills in the areas where the boxes aren’t. Now I could put the background in the actual movie, but since the movie is centered in the browser it won’t match up with the background in the html. Can I make the flash movie backgound transparent?