Thirteen Japanese and five American mother-infant relationships were observed longitudinally during the first year of infants' lives in their homes. There were few infant differences, but maternal behaviors and mother-infant interactions did differ. American mothers vocalized to and touched their infants more, and interactions between American dyads were more than between Japanese dyads. Developmental changes in infant behaviors, maternal behaviors, and mother-infant interaction were observed, understanding the necessity of longitudinal studies of mother-infant interactions, especially across culture.