April 7, 2010 1:46:14 AM EDT
It depends on the texture of the board. If the board shows 99KKx and you have a 9 in your hand and Villian is betting $100 it's quite possible he has the King and has you beat.
It's difficult to tell based on the information you provided. If we knew the action before the flop, on the flop, turn and river, including what cards came, bet amounts, etc. that helps us to paint a picture of the situation and determine what your best course of action is.