Objective: Substitute fathers are often reported to commit child abuse at higher rates than birth (i.e., putative genetic) fathers. Due to the paucity of studies, especially in developing countries, and to some conflicting results from developed countries regarding the identity of perpetrators of less extreme forms of physical abuse of children in stepfamilies, the aim of this study was to assess whether the presence of a stepfather would constitute a risk factor for violence against Brazilian children, as reported by their mothers. Methods: Three hundred eighty five women with a current male partner and a child aged 1-12 years were interviewed. Child physical abuse was assessed using the "Conflict Tactic Scales Parent Child". Data were analyzed through multiple logistic regressions. Results: Physical abuse (including severe physical abuse) was reported for 34.0% of the children with stepfathers (N=54) versus 17.6% of those living with putative genetic fathers (N=331). When adjusted for income, education, ages, whether the child was the first born, number of siblings, number of people in the household, how much time the man spent with the child, how many years he resided with the mother, and alcohol abuse and drug use by the mother and her partner, the adjusted odds ratio was 2.7 (95% CI: 1.2-5.9) for stepfather households over 2 genetic parent households. Surprisingly, the elevated risk to stepchildren was due to alleged abuse by the interviewee herself rather than her partner. Conclusion: Brazilian mothers professed to abuse their own children at substantially higher rates when their male partners were stepfathers to the focal child as compared to genetic fathers. (Contains 3 tables.)