Hi there, anybody could let me know how to create video with transparent background?

I have been using the "croma keying" to remove the video background in Adobe premier 6.0 and saved the movie as quicktime. When i imported it into flash 8 or 7, I noticed that my video background was still there.

Anybody knows whether there is a specific setting or format that i need to follow inorder to make the background transparent?

I would really appreciate it, if anyone can help me asap.

thanks!!