Background. The two vaccination coverage surveys conducted in French Guiana in 1990 and 2000 revealed insufficient pertussis vaccination coverage throughout the department. In the coastal region, vaccination coverage at 24 months with three doses of pertussis vaccine was estimated at 85%. Coverage with four doses of pertussis vaccine was even lower. It was estimated at 43% at 24 months and 77% at 6 years. Vaccination coverage at age 12 remains low in older cohorts; only 60% of adolescents received four doses of pertussis vaccine. These results were confirmed by a vaccination monitoring survey conducted in April 2003 in certain neighborhoods of Cayenne and Rémire-Montjoly, where pertussis vaccination coverage for children aged 6 months to 6 years was found to be insufficient in all surveyed areas. The presence of large groups of infants and children who are inadequately vaccinated against pertussis increases the population susceptible to Bordetella pertussis, consisting of adolescents and adults whose natural or vaccine-induced immunity has waned. This situation amplifies the risk of the bacterium circulating in the population and thus of severe cases occurring among unimmunized infants. Added to this is the occasional occurrence of pertussis epidemics in border areas of neighboring countries (Brazil, Suriname). The most recent one occurred in October 2005 in Brazil. 313 cases of pertussis were reported around Kumaruma, on the Oyapock River, of which 38 were clinically confirmed. A catch-up vaccination campaign was implemented in French Guiana in late October 2005 for border populations. In total, 530 vaccines were administered to residents of the Saint-Georges municipality area. In this context, the occurrence of pertussis outbreaks must be monitored with the utmost care in order to rapidly implement appropriate surveillance and control measures. Alert. On November 22, 2005, and in accordance with the recommendation issued by the French Higher Council of Public Health (CSHPF) regarding the course of action to be taken in the event of clustered cases of pertussis, a private practitioner in Rémire-Montjoly (French Guiana) reported two clustered cases of pertussis to the Health Surveillance Unit (CVS) of the Directorate of Health and Social Development (DSDS) of French Guiana. On November 29, the same physician reported four new cases of pertussis as well as six suspected cases. The Antilles-French Guiana Interregional Epidemiology Unit (Cire A-G) was called upon to assist in assessing the situation. On December 1, Cire A-G and the DSDS of French Guiana forwarded this report to the InVS alert coordination unit and to the Department of Public Health Emergencies at the DGS. A field investigation was conducted in collaboration with the French Guiana Departmental Maternal and Child Health Service (PMI) from December 7 to 9 in the affected municipalities to validate the report. Epidemiological surveillance of pertussis cases was then established by the Health Surveillance Unit, in coordination with private and hospital-based clinical laboratories.