Categorial discrimination tests (CDTs) with equal opportunities for ‘‘hits’’ and ‘‘false alarms’’ assessed native Japanese and English subjects’ perception of English and Japanese segmental contrasts. The stimuli consisted of vowels and consonants spoken at two rates by five native speakers each of English and Japanese. The listeners chose the odd item out in triads, if there was one, or responded ‘‘none’’ if they heard three physically different tokens of a single vowel or consonant. A’ scores were computed for each contrast. The subjects were ten native English (NE) controls, ten ‘‘experienced’’ bilinguals (Japanese speakers who had spent several years in the United States), and ten ‘‘inexperienced’’ bilinguals (Japanese speakers who had never lived outside Japan, but had studied English in school). Analysis of the CDT results will help determine (1) if the subjects differ in their ability to discriminate consonants and vowels, (2) if perceptual sensitivity to vowel and/or consonant contrasts is affected by the speech rate, and (3) if the experienced bilinguals more closely resemble the NE controls than the inexperienced bilinguals do. [Work supported by NIH.]