Reported here is a first study of the implementation fidelity group intervention program for sexually abused adolescent girls, consisting of 18 sessions centered on 9 themes. Through systematic observation of the intervention protocol used with two groups of adolescents, the case study describes implementation fidelity and its evolution. Results show that the first group outperformed the second in terms of implementation adherence. Levels of participation were similar between groups but quality appears to have improved for the second group. The activities implemented lasted longer than planned in both groups. Results are discussed in light of the factors that promote fidelity and the debate between fidelity and adaptation.