This study showed that although mothers' boyfriends perform relatively little child care, they are responsible for substantially more child abuse than other nonparental caregivers. Using data drawn from interviews with single mothers and records of child abuse substantiated through child protection investigation, mothers' boyfriends' overrepresentation in child abuse was traced to five conditions: (a) the location of their child care in single parent families; (b) their gender; (c) the absence of genetic relationship between mothers' boyfriends and their partner's children; (d) mothers' boyfriends' perceived illegitimacy as caregivers and family members; and (e) mothers' boyfriends' rivalry with their partner's children. The limitations of these findings and implications for future research are discussed.