The mismatch negativity (MMN) correlates of the perception of roughness, the unpleasant character of sounds caused by the perception of amplitude fluctuation in the range of 20-200 Hz, were studied on the basis of a variation in the degree of modulation (=modulation index m), which is a main parameter influencing roughness. A psychophysical study showed that perceived roughness of tones increased with modulation index for m-values from 0 up to 1.2. For larger values of m, roughness perception remained stable. In a subsequent ERP-study, infrequent amplitude modulated (AM) tones with varying modulation index were presented in the context of a series of pure tones in an ignore condition. The amplitude of the mismatch negativity correlated highly with the roughness ratings (r = -0.93) and did not increase monotonously with increasing modulation index. We conclude that perceived roughness rather than its physical correlate in sounds is reflected by the MMN and that roughness is thus preattentively encoded.