This study utilized a partially nested randomized control design to investigate the impact of Fusion, a first-grade math intervention. Blocking on classrooms, students were randomly assigned to one of the three conditions: a Fusion two-student group, a Fusion five student group, or a no-treatment control group. Two primary research questions were examined: What was the overall impact of the Fusion intervention as compared to a business-as-usual comparison condition? and Was there a differential impact on student outcomes between the 2:1 Fusion and the 5:1 Fusion conditions? Analyses found a positive effects on four outcome measures favoring Fusion groups over control with two of the differences statistically significant. Results between Fusion groups found positive effects favoring the Fusion 2:1 group compared to the Fusion 5:1 group on all four outcome measures with two of the differences statistically significant. On a second-grade follow-up measure, no difference was found between Fusion groups and control, but a statistically significant difference was found between Fusion groups favoring the 2:1 Fusion group. Future research directions and implications for practice are discussed. [For the corresponding grantee submission, see ED626858.]