This study examines how the entrance of a stepfather influences adolescent ties to mothers and nonresident fathers and how prior ties to each biological parent influence the development of stepfather-stepchild ties. Data come from 1,753 adolescents in the National Longitudinal Study of Adolescent Health who lived with a single mother in Wave 1 who remained single, cohabited, or married by Wave 2, approximately 1 year later. Stepfamily formation had little consequence for adolescent-nonresident father ties. Adolescent-mother closeness, however, declined when cohabiting, but not married, stepfathers entered the household. Close ties to married stepfathers were more likely to develop when adolescents were closer to their mothers before stepfather entry. Prior ties to nonresident fathers were unrelated to stepfather-stepchild ties.