Speech perception may be viewed as a phonetic categorization task in which the listener assigns incoming sounds to various phonetic categories. The present experiment tests two classes of models of phonetic categorization: (1) models in which the listener has a threshold or boundary between alternative categories vs. (2) models in which the listener compares the input to prototypical representations of the alternative phones. In a pretest, listeners located on a VOT continuum the /ga/ that they thought was prototypical. Selective adaptation was then conducted using both the selected prototype and adaptors nearer and further from the phoneme boundary. The prototype adaptor produced more adaptation than the other adaptors. This result supports a prototype-based representation for phonetic categorization; several process models using such a representation are considered.