Previous studies have shown that a frequency change in a continuous tone elicits an NI type of ERP (event-related potential) component. It remained unclear, however, whether this response is a "genuine" N1 (onset detector response) or the mismatch negativity (MMN), a change-detector type of ERP response, elicited in previous studies by an infrequent change in a sequence of homogeneous stimuli. A further possibility is a nearly perfect overlap of the two types of ERP components. The advent of modern, high-resolution magnetometers has opened a new, powerful way to tackle such component-overlap problems. Subjects were presented with a continuous tone of 988 Hz which was occasionally increased to 1108 Hz for a period of 100 msec. The magnetic responses to this change consisted of two partially overlapping components with peaks separated by 30 msec. The earlier component was probably generated by neuronal populations of the auditory cortex corresponding to the supratemporal N1, whereas the later one, generated anteriorly and inferiorly to the first, probably reflects a mismatch process causing the magnetic equivalent of the electrical MMN.