Preliminary data from the Fragile Families survey were analyzed to examine the impact of incarceration on marriage and family structure. The study specifically considered the characteristics of married and unmarried fathers at risk of imprisonment. Men who have been incarcerated are less likely to be married than men who have never been in prison. Former inmates were also less likely to have a relationship with the mother of their child at the child's first birthday. Regression analysis with incarceration as a dummy variable supported the finding that a history of incarceration negatively affects the likelihood of cohabitation one year after the birth of a couple's child. White and Hispanic couples are more likely than black couples to be living together a year after their child is born and older mothers are more likely to be living with their child's father than younger women. Age and education of the father had no effect on cohabitation or marriage. Incarceration affected couples with weaker relationships to a greater degree than couples with stronger commitments. The implications of this study for research with men at risk of incarceration are discussed. 20 references and 5 tables.