In matching-to-sample, comparison choice should be controlled by the identity of the sample and, when the sample is not available, by the overall probability of reinforcement associated with each of the comparisons. In the present research, pigeons were trained to match a frequent sample (appearing on 80% of the trials) to one comparison (C(fr)) and an infrequent sample (appearing on 20% of the trials) to the other (C(inf)), with the number of reinforcements associated with each sample equated. In Experiment 1, the task was identity matching; in Experiments 2 and 3, it was symbolic matching. We asked whether, when control of comparison choice by the sample was reduced (by inserting a delay between the sample and the comparisons), pigeons would choose comparisons on the basis of (1) the number of reinforcements per comparison (and thus show no comparison bias), (2) the comparison associated with the more frequent sample during training (and show a preference for C(fr)), or (3) the probability of reinforcement given a correct response (and show a preference for C(inf)), or (4) inhibition produced by nonreinforced choice of the more frequently correct comparison (and show a preference for C(inf)). Pigeons showed a significant tendency to choose C(fr). In Experiment 3, we showed that this bias did not result from the effects of intertrial facilitation or interference. Thus, it appears that when control of comparison choice by the sample is reduced, pigeons' choice is controlled not merely by the probability of reinforcement but also by overall sample frequency.