Purpose: The current study aims to examine the influence of contextual and provider-level factors on the implementation fidelity of a research-based substance abuse prevention program. Also, it aims to investigate whether two provider-level factors, self-efficacy and beliefs about the value of the program, statistically moderate and mediate the effects of a provider training intervention on implementation fidelity. Design/methodology/approach: Using generalized mixed-linear modeling, the authors examine relationships between program provider-, organizational, and community-level factors and implementation fidelity in a sample of 50 high school teachers from 43 high schools in eight states across the USA. Fidelity of implementation was assessed utilizing an observation procedure. Findings: Implementation fidelity was negatively associated with the urbanicity of the community and the level of teachers' beliefs about the value of the program, and positively predicted by the organizational capacity of the school. Comprehensive training significantly increased teachers' self-efficacy, which resulted in an increase in implementation fidelity. Research limitations/implications: School-based prevention program implementation is influenced by a variety of contextual factors occurring at multiple ecological levels. Future effectiveness and dissemination studies need to account for the complex nature of schools in analyses of implementation fidelity and outcomes. Practical implications: The authors' findings suggest that both provider- and organizational-level are influential in promoting implementation fidelity. Before implementation begins, as well as throughout the implementation process, training and ongoing technical assistance should be conducted to increase teachers' skills, self-efficacy, and comfort with prevention curricula. Originality/value: The present study is one of the few to examine contextual and provider-level correlates of implementation fidelity and use mediation analyses to explore whether provider-level factors mediate the effects of a provider training intervention on implementation fidelity. (Contains 2 tables and 2 figures.)