QuickTime player itself is limited, even with QT Pro. As LW newbie suggested you need to get extra codecs. Unfortunately DivX cannot play all AVI files. Neither can the DivX player. Better as suggested by FOTH is VLC player. I have my VLC set as the default to play avi files & some other formats, & QT Player to play mpeg 2 files. See how to change default program here How to change default program in OSX