Old World monkeys were trained with an operant conditioning technique to discriminate the natural speech sounds /ba/–/da/ and transferred to synthetic speech. Human and monkey difference thresholds for formant transitions were then compared along a seven‐step /ba/–/da/ continuum. Monkeys were not as sensitive as humans to differences in formant transition: the just noticeable difference for monkeys was about 320 Hz, and for humans, about 160 Hz. Although humans were more adept at intraphonemic discriminations than monkeys, their latencies to stimulus changes revealed evidence of ’’categorical perception’’ of the continuum: While latencies for the monkeys increased linearly as stimulus difference was decreased, human latencies were essentially constant for all interphonemic comparisons, but increased sharply for intraphonemic comparisons. We view these data as evidence for (a) similar sensory capacities in monkeys and humans, but (b) unique speech processing capacities in humans.Subject Classification: [43]65.75; [43]70.30; [43]80.50.