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Thirty-two undergraduates were exposed to a fixed-interval 60-s schedule. There were extreme individual differences in postreinforcement pauses and response rates. Such individual differences persisted for 60 sessions with a total of 1200 reinforcers. The total number of errors made on the Matching Familiar Figures Test was negatively correlated with the postreinforcement pause and positively correlated with the response rate. These results demonstrate that individual differences in human fixed-interval performance are robust and reliable, and relate to impulsiveness.