Fatherhood has traditionally been viewed as part of a “package deal” in which a father’s relationship with his child is contingent on his relationship with the mother. We evaluate the accuracy of this hypothesis in light of the high rates of multiple-partner fertility among unmarried parents using the Fragile Families and Child Wellbeing Study, a recent longitudinal survey ofnonmarital births in large cities. We examine whether unmarried mothers’ and fathers’ subsequent relationship and parenting transitions are associated with declines in fathers ’ contact with their nonresident biological children. We find that father involvement drops sharply after relationships between unmarried parents end. Mothers’ transitions into new romantic partnerships and new parenting roles are associated with larger declines in involvement than fathers’ transitions. Declines in fathers’ involvement following a mother’s relationship or parenting transition are largest when children are young. We discuss the implications of our results for the well-being ofnonmarital children and the quality of nonmarital relationships faced with high levels of relationship instability and multiple-partner fertiliy.