Eighty-one 6-month-old infants and their mothers were videotaped in Tronick's face-to-face still-face paradigm to evaluate gender differences in infant and maternal emotional expressivity and regulation. Male infants had greater difficulty than female infants in maintaining affective regulation during each episode, including the still face. Mother-son dyads had higher synchrony scores than mother-daughter dyads but took longer in repairing interactive errors. In addition, maternal affect, matching, rate of change between matching and mismatching states, and synchrony in the play preceding the still face differentially mediated male and female infants' responses to the still face and reunion play. The developmental implications of these gender differences are discussed.