This study investigated whether individual differences in working memory (WM) moderate effects of 2 variations of intervention designed to improve at-risk 4th graders' fraction knowledge. We also examined the effects of each intervention condition against a business-as-usual control group and assessed whether children's measurement interpretation of fractions mediated those effects. At-risk students (n = 243) were randomly assigned to control and 2 intervention conditions. The interventions each lasted 12 weeks, with three 30-min sessions per week. The major focus of both intervention conditions was the measurement interpretation of fractions. Across the 2 conditions, only 5 min of each 30-min session differed. One condition completed activities to build fluency with 4 measurement interpretation topics; in the other, activities were completed to consolidate understanding on the same 4 topics. Results revealed a significant aptitude-treatment interaction, in which students with very weak WM learned better with conceptual activities but children with more adequate (but still low) WM learned better with fluency activities. Both intervention conditions outperformed the control group on all outcomes, and improvement in the measurement interpretation of fractions mediated those effects.