Background: Extraordinary events (KLB) diphtheria are still found in the city of Banda Aceh. An action is needed to break the chain of diphtheria cases and reduce the number of diphtheria morbidity or mortality, one of which is by conducting an epidemiological investiga-tion of diphtheria. This study aims to see how the implementation of diphtheria epidemiology investigations in the city of Banda Aceh and how it relates to the presence of diphtheria cases in the city of Banda Aceh. Subjects and Method: This was a qualitative multiplecase study. Data were collected by conducting interviews with doctors in five sample community health center in Banda Aceh City. Community health centers were selected by purposive sampling based on primary data regarding the distribution of diphtheria during 2017 and 2018 in Banda Aceh City. This study combined source tri-angulation. The source triangulation informant is surveillance who served in the Banda Aceh City Health Service. Results: The epidemiological investigation effort at the puskesmas began by immediately sending a diphtheria fast-moving team to carry out a chronological-related survey of cases and collect data on close contacts in the case environment. In close contact data collection there are community health center that do not record complete close contact so prophylaxis cannot cover all close contacts. The stage of examination of throat or nose specimens for close contact is also not carried out in some community health center. Conclusion: Epidemiological investigations play an important role in breaking the chain of diphtheria distribution. Skipping stages in epidemiological investigations can provide a gap for the development of diphtheria in the case environment. Puskesmas that carry out a complete stage of epidemiological investigation are proven to be able to eradicate the incidence of diphtheria in their working area.