Discrimination of speech-sound pairs drawn from a computer-generated continuum in which syllables varied along the place of articulation phonetic feature (/b,d,g/) was tested with macaques. The acoustic feature that was varied along the two-formant 15-step continuum was the starting frequency of the second-formant transition. Discrimination of stimulus pairs separated by two steps was tested along the entire continuum in a same-different task. Results demonstrated that peaks in the discrimination functions occur for macaques at the "phonetic boundaries" which separate the /b-d/ and /d-g/ categories for human listeners. The data support two conclusions. First, although current theoretical accounts of place perception by human adults suggest that isolated second-formant transitions are "secondary" cues, learned by association with primary cues, the animal data are more compatible with the notion that second-formant transitions are sufficient to allow the appropriate partitioning of a place continuum in the absence of associative pairing with other more complex cues. Second, we discuss two potential roles played by audition in the evolution of the acoustics of language. One is that audition provided a set of "natural psychophysical boundaries," based on rather simple acoustic properties, which guided the selection of the phonetic repertoire but did not solely determine it; the other is that audition provided a set of rules for the formation of "natural classes" of sound and that phonetic units met those criteria. The data provided in this experiment provide support for the former. Experiments that could more clearly differentiate the two hypotheses are described.