Multilevel modeling was used to investigate the relationship between poverty status, mathematics achievement gains, and behavioral engagement in learning over kindergarten. Data included information on 11,680 poor, low-income, and non-poor kindergartners from the Early Childhood Longitudinal Study-Kindergarten Cohort of 1998-1999 (ECLS-K). Results show that accounting for teachers' reports of non-poor students' higher behavioral engagement (e.g., attentiveness, task persistence) explains the disparity in mathematics gains between poor and non-poor students over kindergarten. Furthermore, results suggest that exposing poor and low-income students to higher classroom behavioral engagement could play a substantial role in equalizing mathematics gains. Given these findings, strategies designed at narrowing economic disparities in early mathematics achievement should take into consideration the impact of behavioral engagement at the intrapersonal and contextual levels. (Contains 3 notes and 4 tables.)