This study examined the efficacy of a kindergarten mathematics intervention program, ROOTS, focused on developing whole-number understanding in the areas of counting and cardinality and operations and algebraic thinking for students at risk in mathematics. The study utilized a randomized block design with students within classrooms randomly assigned to treatment or control conditions. Measures of mathematics achievement were collected in the fall (pretest) and spring (posttest) in kindergarten and in the winter of first grade (delayed posttest). Significant differences between conditions favoring treatment students were found on four of six measures at posttest. Treatment students reduced the achievement gap with their not-at-risk peers. No effect was found on follow-up first-grade achievement scores. Implications for Tier 2 mathematics instruction in a Response to Intervention model are discussed. [At time of submission to ERIC this article was in press with the "Journal of Research on Educational Effectiveness."]