Does a creature with improved trip (like a wolf) have to make a separate CMB check after the initial attack?

[Garygax]

Thanks for the clarification.

[harmor]

[b wrote:

Trip (Ex)[/b]]

A creature with the trip special attack can attempt to trip its opponent as a free action without provoking an attack of opportunity if it hits with the specified attack. If the attempt fails, the creature is not tripped in return.

Format: trip (bite); Location: individual attacks.

Looks like you do, but you may want to house-rule it to use the same roll (one less roll to make - kinda like how a Shield of Bashing would work).