the first season it was the middleweights 170-185 and the light heavyweights 185-205. they break down the weight classes like this.

Lightweight - over 145 lbs. to 155 lbs.
Welterweight - over 155 lbs. to 170 lbs.
Middleweight - over 170 lbs. to 185 lbs.
Light Heavyweight - over 185 lbs. to 205 lbs.
Heavyweight - over 205 lbs. to 265 lbs.