It is well established that young infants process speech in terms of perceptual categories that closely correspond to the phonetic categories of adult language users. Recently, Kuhl (1991) has provided evidence that this correspondence is not limited to the region of category boundaries: At least by 6-7 months of age, vowel categories of infants, like those of adults, have an internal perceptual structure. In the current experiments, which focused on a consonantal contrast, we found evidence of internally structured categories in even younger infants-3-4 months of age. The implications of these findings for the nature ofthe infant's earliest language-universal categories are discussed, as is the role of exposure to the native language in shaping these categories over the course of development.One of the fundamental issues in speech perception is how listeners map the continuously varying signal of speech onto the phonetic categories of their language. Much of the research on phonetic categorization over the years has focused on the boundaries between phonetic categories, addressing such issues as why boundaries fall where they do along given acoustic continua, what kinds ofcontextual factors playa role in determining boundary location, and the effects of specific linguistic experience in altering boundary location over the course of development (see, e.g., Repp & Liberman, 1987). This emphasis on boundaries derived in part from the much-studied phenomenon of categorical perception, which emphasized the relative difficulty experienced by listeners in discriminating members of a given phonetic category compared with the ease in discriminating tokens that cross a category boundary (Repp, 1984;Studdert-Kennedy, Liberman, Harris, & Cooper, 1970).It has become increasingly clear, however, that phonetic categories are much richer in form than this emphasis on boundaries would suggest. It is now known that listeners can discriminate stimuli from a given category with a relatively high degree of accuracy, given specific stimulus and task conditions (Carney, Widin, & Viemeister, 1977;Samuel, 1977; see also Macmillan, Goldberg, & Braida, 1988). Moreover, and of particular relevance for the current research, there is a growing literature indicating that phonetic categories have a graded internal struc-