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What is the difference between quicktime 6.5.2 and quicktime pro?

Quicktime 6.5.2 is merely a version number. Quicktime Pro is the enhanced copy of the program which has been available for all the versions. It adds more capabilities, like saving files in different formats with a variety of options and codecs. You need to purchase the upgrade to Quicktime from the Apple Store. They will give you a code to unlock the "pro" features of the software.

Eric: I've renamed your thread to say what you are looking for

If I'm not mistaken the Pro version (at least on the PC) also allows
fullscreen playback (which the normal version doesn't do).

That's true Rob. There are many added features in the pro version. See the "top ten list" on Apple's website here http://www.apple.com/quicktime/download. For $30 it's a really useful upgrade from the free version. QT Pro is also included with Final Cut Pro (and I assume Final Cut Express as well).

Quicktime Pro is super useful (at least on a Mac). Import / export tracks, resize things, merge files end to end or split them up, etc. I can't think of one specific clincher that makes QT Pro so great, but rather it's all the extra little things you can do with it.

QT Pro on a PC is probably equally useful with mov content, but I've never really used it under Windows. AFAIK QT doesn't support AVI files (it does on Mac) so it's kind of limited in usefulness.