Employing a multi-method approach, we investigated observed and parent-rated child behavioral inhibition (BI) and maternal reports of their own negative affectivity (NA) as predictors of young children's internalizing problems. Participants were 201 children who were siblings between 2.5 and 5.5 years of age (mean = 3.86, standard deviation = 1.04) and their mothers. Due to the nested research design, multilevel model-fitting analyses were used to examine associations between predictors and internalizing problems, and to test a mediational process between maternal NA and internalizing problems. High levels of both observed and parent-rated child BI and greater maternal NA significantly predicted internalizing problems. Child age also moderated the association between parent-rated child BI and internalizing problems, with the association stronger for younger and mean age children, but not for older children. Additionally, parent-rated child BI was found to mediate the association between maternal NA and internalizing problems, and the mediational effect was stronger for younger children.