Background: Child care centers (CCC) can be strategic settings to establish healthy lifestyle behaviors through obesity prevention programs. Fidelity to the implementation of such programs is a vital evaluation component, but is often not measured. This study assessed CCC teacher fidelity to the implementation of “Healthy Caregivers, Healthy Children (HC2)”, a CCC-based obesity prevention intervention. Methods: CCCs serving low-resource, ethnically diverse families with ≥ 50 children ages 2-to-5 years old that were randomized to the HC2 intervention and that had teacher fidelity data collected (n = 9 CCC) were included in this analysis. The Environment and Policy Assessment and Observation (EPAO) tool assessed the CCC nutrition and physical activity (PA) environment at the beginning/end of the school year. Fidelity assessments were conducted in CCCs randomized to HC2 in Spring 2016 (n = 33 teachers) and 2017 (n = 39 teachers) by a trained observer. The relationship between teacher fidelity and EPAO was assessed via mixed models. Results: For every-one unit rise in teacher fidelity, EPAO nutrition increased 0.055 points (p =.006). No significant relationship was shown between teacher fidelity and EPAO PA score (p =.14). Conclusion: Teacher fidelity to obesity prevention program implementation may support a healthy CCC obesity prevention and nutrition environment but might require continued support for all components. (author abstract)