[TYPO3-english] Blocking a page for a usergroup

[Disketten Charly]

Hello,

This is doing my head in. Here's my page structure:

Usergroup A
Usergroup B

Page A
Page B

I'd like to achieve this:
Without login => only Page A
Usergroup A logged in: only Page A
Usergroup B logged in: only Page B

Any ideas how to get there? Is it possible to negate the fe user 
permissions, to block Page A for Usergroup B? Or is it possible to have 
a "Usergroup" assigned to every visitor, even without being acutally 
logged in?

Thank you so much!