Reduced supportive parenting and elevated negative parenting behaviors increase risks for maladaptive social adjustment during early childhood (e.g., Campbell, Shaw, & Gilliom, 2000). However, the magnitude of these risks may vary according to children's individual characteristics, such as sex and temperament. The current study examines whether children's sex and fear reactivity moderate the associations between mothers’ observed parenting and children's behavior problems 1 year later. The sample consists of 151 predominantly African American, low-income families with one sibling who is approximately 2 years old and the closest aged older sibling who is approximately 4 years old. Results from fixed-effects within-family models indicate that fear distress (i.e., fearfulness) moderated associations between mothers’ observed negative parenting and children's increased behavior problems, such that only those children with mean or higher observed fear distress scores showed increased behavior problems when exposed to mother's negative parenting. Child sex moderated associations between fear approach reactivity (i.e., fearlessness) and mothers’ observed supportive parenting. Specifically, low fear approach combined with supportive parenting was associated with fewer behavior problems for boys only. Implications of these findings for preventive intervention are discussed.