We investigated the emergence of discriminative responses to pitch by recording 2-, 3-, and 4-month-old infants' electro-encephalogram responses to infrequent pitch changes in piano tones. In all age groups, infants' responses to deviant tones were significantly different from responses to standard tones. However, two types of mismatch responses were observed simultaneously in the difference waves. An increase in the left-lateralized positive slow wave was prominent in 2-month-olds, present in 3-month-olds, but insignificant in 4-month-olds. A faster adultlike mismatch negativity (MMN), lateralized to the right hemisphere, emerged at 2 months of age and became earlier and stronger as age increased. The coexistence and dissociation of two types of mismatch responses suggests different underlying neuromechanisms for the two responses. Furthermore, the earlier emergence of the MMN-like component to changes in pitch compared to other sound features implies that neural circuits involved in generating MMN-like responses have different maturational timetables for different sound features.