We replicated a study of a kindergarten mathematics intervention, ROOTS, delivered in the context of a research-based core program. We randomly assigned 62 classrooms to treatment (ROOTS) or a business-as-usual control. All classrooms implemented the research-based core program (Early Learning in Mathematics). Participants included 163 treatment students and 145 control students nested within classrooms. Key differences between the current replication study and the original study included geographical region, instructional context, and student initial skill. In contrast to the significant positive effects (Hedges's g values of 0.30 to 0.38) found in the original study, no significant differences were found between the treatment and control conditions in this study (Hedges's g values of -0.09 to 0.12) Pretest skills did not moderate treatment effects. We discuss these results' implications for replication research and evaluating intervention efficacy. [For the corresponding grantee submission, see ED618901.]