- Implementation climate is the most proximal organizational predictor of implementation success. First-level supervisors are in a pivotal position to influence implementation climate, as they directly oversee frontline staff who deliver evidence-based interventions (EBIs). First-level supervisors who exhibit strong leadership and utilize best practices in supervision may be able to establish a strong implementation climate. This study aimed to develop and test a middle-range theory exploring 1) first-level supervisor implementation climate influence on first-level supervisor behaviors (i.e., supervision & implementation leadership), and, 2) the dual role of first-level supervisors in shaping caseworker (i.e., frontline staff) implementation climate within the child welfare system (CWS). The theory, which expands on Aarons and colleagues’ organizational framework (2012), was examined using data from a study assessing the implementation of R3, a CWS supervisor-targeted intervention to promote the effective use of EBIs (N = 91 supervisors; 331 caseworkers). A hierarchical linear modeling approach was employed to analyze all paths of the theoretical model. Single timepoint predictors at baseline and one-year later were modeled to estimate associations with change in outcome variables (i.e., supervisor fidelity to R3, supervisor implementation leadership, caseworker implementation climate) over time. Descriptively, first-level supervisor perceptions of implementation climate did not demonstrate a strong correlation with the caseworkers whom they supervised (e.g., r(70) = .119, p > .319), suggesting distinct perceptions of implementation climate are possible. Results support the hypotheses that effective supervision (i.e., fidelity to R3) and implementation leadership significantly increase caseworker implementation climate (e.g., β = .212, p = .010 & β = .228, p = .030 respectively). Further, first-level supervisor implementation leadership behaviors at baseline were associated with a substantial increase in fidelity (β = .196, p < .001). First-level supervisor implementation climate was associated with an increase in fidelity. Results from path analyses between first-level supervisor implementation climate and implementation leadership were inconclusive. In summary, the findings that first-level supervisors serve a dual role in shaping implementation climate supports the theoretical model, and, is a novel contribution to the field. This finding suggests that training initiatives to improve supervision and leadership have the potential to impact implementation success. Further research is needed to understand 1) the antecedent(s) that influence implementation leadership, and, 2) how first-level supervisors support caseworkers even when the organization implementation climate is poor.