Evidenced-based mathematics interventions are critical for supporting students with mathematics difficulties. In research and practice, collecting implementation fidelity is important for ensuring that all the core components of the intervention are implemented as designed. Historically, implementation fidelity has been defined as multi-faceted including examinations of adherence, instructional quality, and student engagement, though mathematics intervention studies rarely report on fidelity components outside of adherence. The current study examined relationships between these different components of fidelity and whether they are associated with student mathematics outcomes and intervention group size within the context of a first-grade mathematics intervention. Findings revealed relationships between components of fidelity with student initial mathematics skill, however no relationship was observed between fidelity components and student mathematics growth. Findings for group size were mixed. Limitations, implications for research and practice, and future directions are discussed. [This paper will be published in "Journal of Learning Disabilities."]