Young infants discriminate both native and nonnative phonetic contrasts, but 10- to 12-month-olds and adults fail to discriminate some nonnative contrasts. To explain this, Best, McRoberts, and Sithole (1988) hypothesized that at the age of 10-12 months, a phonological influence begins by means of which nonnative sounds are assimilated to native categories when possible, and are perceived in auditory or nonspeech terms when they are not assimilable. Best et al. tested English-language infants and adults on discrimination of a Zulu click contrast that was not assimilable to English phonological categories. As predicted, the clicks were discriminated up to 14 months and even into adulthood, suggesting that the perceptual reorganization toward the end of the first year reflects phonological development. The study by Best et al. had not directly compared single-category assimilable and non-assimilable contrasts. The present study compared English-learning infants' discrimination of the Zulu clicks and of a single-category assimilable contrast, the Thompson glottalized stops, and an English stop contrast. As expected, 6- to 8-month-olds discriminated the English and nonnative contrasts. However, 10- to 12-month-olds discriminated only the Zulu and English contrasts. These findings strengthen the evidence that the perceptual reorganization that occurs by 10-12 months reflects phonological development. (RH)