Identifies factors associated with multiple-partner fertility. Almost one third of fathers who reported multiple-partner fertility did so across a series of nonmarital relationships, and nonmarital-only multiple-partner fertility has been increasing across recent cohorts of men. Being older, having a first sexual experience or a first child at a young age, and fathering a child outside of marriage or cohabitation are associated with greater odds of multiple-partner fertility, whereas having additional children with the first birth mother is associated with reduced odds. Black, Hispanic, and young fathers have especially high odds of experiencing multiple-partner fertility across a series of nonmarital relationships.