A new series of studies on adults' and infants' perception of phonetic "prototypes," exceptionally good instances of phonetic categories, show that prototypes play a unique role in speech perception. Phonetic category prototypes function like "perceptual magnets" for other stimuli in the category. They attract nearby members of the category, rendering them more perceptually similar to the category prototype than would be expected on the basis of physical distance alone. Nonprototype stimuli from the category do not function in this way. Moreover, by 6 months of age, infants tested in the United States and Sweden show that the perceptual magnet effect is language specific. Infants from the two countries exhibit the magnetic effect only for the phonetic prototypes of their own language. Thus, exposure to a specific language alters infants' perception of speech by 6 months of age. These results offer an explanation for the findings of a variety of studies on cross-language speech perception in infants and adults, have implications for second-language learning, and are consistent with data on the representation of cognitive categories outside the domain of speech. The results support a new model which describes how innate factors and experience with a specific language interact in the development of speech perception. (Author)