The longitudinal contributions of emotion regulation and emotion lability-negativity to internalizing symptomatology were examined in a low-income sample (171 maltreated and 151 nonmaltreated children, from age 7 to 10 years). Latent difference score models indicated that for both maltreated and nonmaltreated children, emotion regulation was a mediator between emotion lability-negativity and internalizing symptomatology, whereas emotion lability-negativity was not a mediator between emotion regulation and internalizing symptomatology. Early maltreatment was associated with high emotion lability-negativity (age 7) that contributed to poor emotion regulation (age 8), which in turn was predictive of increases in internalizing symptomatology (from age 8 to 9). The results imply important roles of emotion regulation in the development of internalizing symptomatology, especially for children with high emotion lability-negativity. (Contains 2 tables and 2 figures.)