Mutual regulation during the naturalistic interaction of 150 mothers and their 4-month-old infants was investigated from a dynamic systems perspective. Microanalyses of a wide range of behaviors and analysis of contingencies indicated that a 3-s time period best captured contingencies. Both mothers and infants communicated primarily through vocal signals and responses, although maternal touches and infant looks also elicited responses. Although more expressive mothers did not have infants who behaved similarly, levels of contingent responsiveness between partners were significantly associated and occurred within distinct behavioral channels, suggesting coregulated interactional processes in which contingently responsive mothers shape their infants' communications toward mutual similarity. Mothers were more influential than infants over object play, whereas infants were more influential than mothers over expressive behavior. Interactional context consistently influenced contingent responsiveness; there was less mutual responsiveness when the infant was exploring, being held, or looking.