To measure the symmetry of adult categorical phoneme perception, 10 adult male undergraduate students enrolled in an introductory psychology class were the subjects for this study as part of their course requirements. The stimuli used in this study were prepared at Haskins Laboratories by means of a parallel resonance synthesizer and computer. The bilabial stop consonants generated had voice onset times of 0, 20, 40, 60, and 80 milliseconds. Each consonant was followed by the vowel "a." Each subject was tested individually in a sound dampened room. Subjects were seated at a table facing two speakers, one located 90 inches from the subject, the other located 6 inches directly behind the first. A response key was positioned directly in front of the subject. The results replicated those of Liberman and other researchers (1967) which demonstrated that although voice onset time is a continuum, adult perception along that continuum appears categorical. (SW)