An experiment was made to determine if the discriminability of an acoustic difference between speech sounds depends on whether or not that difference cues a phonemic distinction. For comparison, discrimination data were also obtained for control stimuli in which analogous differences occurred in sounds that are not perceived as speech. The speech sounds, which were synthetic approximations to /do/ and /to/, differed only in the relative time of onset of the first and second formants. Discrimination of these sounds was found to be considerably better across the phoneme boundary than in the middle of the phoneme category. Indeed, this effect was so marked as to justify the assumption that the perception of these stimuli is essentially categorical. The control stimuli were produced by inverting the speech patterns on the frequency scale. There was, for these stimuli, no increase in discriminability in the region corresponding to the location of the phoneme boundary and, in general, their discriminability was...