We report on a practitioner implementation of Fusion, a first grade mathematics intervention. Studies such as this evaluation of a loose implementation under realistic conditions are important to curriculum developers' understanding of how evidence-based programs and tools work under a variety of implementation scenarios. In this quasi-experimental study, first-grade students (n = 253) in ten schools were assigned to treatment (n = 154) or control (n = 99). Rather than randomly assigning students, schools assigned those students most at risk to treatment and, typically, those less at risk to control. School staff administered pre and post assessments and led Fusion sessions approximately 30 minutes per day, three days per week. The intervention resulted in a significant positive effect on a researcher-developed first grade math measure. The implementation of Fusion and feedback from school staff provided insights to guide the curriculum development process. [This paper was published in "Learning Disability Quarterly" v40 n4 p211-224 Nov 2017 (EJ1156543).]