In January 1998, the Departmental Directorate of Health and Social Affairs (DDASS) of Seine-Maritime reported to the National Public Health Network (the Institute for Public Health Surveillance since March 9, 1999) a significant increase in the number of meningococcal infections (MI) reported in the department in 1997. At the same time, the National Reference Center for Meningococci and Related Neisseria (CNRM) reported the presence in patients from Seine-Maritime of a strain rarely isolated in France and belonging to the ET-5 electrophoretic complex, strain B:14:P1.7,16. In this context, we conducted a detailed analysis of the epidemiological data available through mandatory reporting (DO) and the CNRM’s microbiological data to determine whether the situation warranted the adoption of specific surveillance and prophylaxis measures.