College students made observing responses in a discrete-trial instrumental-conditioning situation. Intertrial intervals from 1 to 10 sec. were factorially combined with patterns of reinforcement involving different total numbers of non-reinforcements, numbers of successively occurring non-reinforcements, and numbers of non-reinforced—reinforced trial transitions. In agreement with previous studies with rats, Exp. 1 indicated that intertrial interval interacts with pattern of reinforcement and accounts for a large percentage of the total variance. Contrary to previous studies with rats, Exp. 2 indicated that the effects of intertrial interval with humans are due to more than just those intertrial intervals near a non-reinforced—reinforced trial transition. Though much of the basic human and animal partial-reinforcement data are similar, the theoretical accounts apparently should differ.