Guided by a microanalytic approach to the study of relationships, we assessed parent, infant, and coparental behaviors during triadic interactions in 94 parents and their 5-month-old firstborn child. Relational behaviors in each family subsystem--mother-infant, father-infant, and coparenting--were microcoded. Marital satisfaction and infant temperament were self-reported. No differences were found in the infants' behavior toward mother and father or in the time spent with each parent. Mothers' and fathers' relational behavior during parent-infant episodes were generally comparable, yet mothers vocalized more and the latency to father's displaying positive affect was longer. Conditional probabilities indicated that under conditions of coparental mutuality, fathers showed more positive behaviors than mothers. Lag-sequential analysis demonstrated that change in the infant's social focus between parents followed change in coparental behavior. Fathers' coparental mutuality was independently predicted by maternal behavior during mother-child episodes, father marital satisfaction, and infant difficult temperament, whereas mothers' coparental mutuality was only linked with fathers' relational behavior. Results highlight the importance of including a microlevel perspective on the family system at the first stages of family development.