not England, but when I go to exercise classes at the gym, they talk about buttocks/butt... or gluteals if referring to the muscles....

I am from England and I think you'd be fine with "bottom". Never been to a gym/exercise class in my life, though, so someone might be able to come up with something more appropriate!

derrière, (those lovely) behinds (or just "behind" if you're talking to one person), butt
I also think butt is acceptable in a gym situation

Or tush

How about "bum"? [Does anyone remember that advert with Anthony Hopkins (I think it was for Barclays)... "Does my bum look big in this?" :-)]

I'd go with "bum" if the class consists of young people and with "bottom", "behind" or "buttocks" if it's a group of elderly people...

However, I would not use "butt" in a professional environment.

#4: tush (or tushy ;-)) would be American English.

http://www.google.de/#q=gym+%22stick+your+but...
http://www.google.de/#q=gym+%22stick+your+bum...
I guess Lucano can read up about which expression he/she wishes to use

I'd favour 'bottom'; must be elderly :-)

LOL CM2DD ... me too :)

#5, most of the classes I go to are mixed age... from 20s to 60s, and it's the professional instructors who use butt/buttock....
- perhaps it's a cultural divide

I'm a bottom(s) man too ;-)

LOL, sorry guys, I didn't want to discriminate anyone with my comment ;-)

Do you agree that tush, bum and butt are more colloquial expressions (with tush and bum being a little "nicer/friendlier" than butt) and that buttocks, behind and bottom are more formal?

PS: The classes I go to usually range from 10 to 67! ;-)

As a BE speaker of a certain age:

butt: American, status uncertain
tush: never heard it
bum: childish
bottom: neutral, the general term
buttocks: verging on the medical
behind: coy
seat: even coyer

Definitely 'bottom' at all the exercise classes I have ever been to, nice and general and non-offensive