Sex differences in rates of behavior problems, including internalizing and externalizing problems, begin to emerge during early childhood. These sex differences may occur because mothers parent their sons and daughters differently, or because the impact of parenting on behavior problems is different for boys and girls. In this study, we examined whether associations between observations of mothers' positive and negative parenting and children's externalizing and internalizing behaviors vary as a function of child sex. The sample consisted of 137 African American low-income families with one sibling approximately 2 years old and the closest-aged older sibling who was approximately 4 years old. Results from fixed-effects within-family models indicate clear sex differences regardless of child age. Mothers were observed to use less positive parenting with sons than with daughters. Higher levels of observed negative parenting were linked to more externalizing behaviors for boys, whereas lower levels of positive parenting were linked to more externalizing behaviors for girls. No child sex differences emerged regarding associations between observed positive and negative parenting and internalizing behaviors.