Categorical perception of certain speech and speech-like sounds has been critical to models of speech perception for decades. Furthermore, refining our knowledge of the stimulus and task characteristics that lead to categorical perception continues to be an important part of research in speech perception. Auditory evoked potentials (AEPs) have the capability of providing new perspectives on this important phenomenon. The mismatch negativity (MMN) AEP is elicited in response to a physically deviant stimulus occurring in a series of standard stimuli [Näätänen etal., Acta Psychol. 42, 313–329 (1978)]. The MMN has been shown to be sensitive to fine acoustic differences and was used in the present study to examine a stimulus continuum that typically leads to categorical perception. The primary auditory cortex is thought to be a major source contributing to the generation of the MMN. In the present experiment a stimulus continuum ranging from /da/ to /ga/ was synthesized by varying the frequencies of F2 and F3. An identification task showed the expected steep slope in the identification function along this continuum, indicating that they were perceived categorically. An electrophysiologic index of discrimination was obtained by recording the MMN in adults to stimulus pairs, having equal acoustic differences, within and across categories. The MMN was observed in all subjects and was the same size both across and within categories. That is, the MMN indicated equal discrimination both across and within categories. These results suggest that the MMN reflects the processing of auditory aspects of these speech-like signals, but not processing into phonetic categories.