In August 2007, three cases of Legionnaires’ disease, involving residents of three neighboring municipalities, were reported to the Pas-de-Calais Departmental Directorate of Health and Social Affairs. An active case investigation was launched. The environmental investigation explored all potential sources of contamination, with a particular focus on cooling towers. The clinical and environmental Legionella strains isolated were typed at the National Reference Center. Nine cases of Legionnaires' disease caused by Legionella pneumophila serogroup 1 were recorded. The onset of symptoms ranged from August 19 to September 5, 2007. The average age of the cases was 77 years. The only common factor among all cases was exposure to an area with a 6-km radius centered on Courrières. Two different clinical strains were isolated. No Legionella was detected on the TARs present in the central zone. Legionella was detected in high concentrations on the wastewater treatment facilities and a nitrification reactor (Circox) at an industrial site. A strain identical to that found in one of the patients was detected at various locations there. A model of water vapor dispersion from the Circox reactor showed good agreement between its geographic location, wind directions during the period, and the distribution of locations frequented by the cases. The investigations pointed to the facilities at the industrial site as the likely source of the outbreak. The hypothesis that the nitrification reactor (Circox) was the primary dispersal factor is highly plausible. This outbreak serves as a reminder that, while wastewater treatment plants (WWTPs) remain the primary sources of Legionella dispersal leading to outbreaks, any facility conducive to their proliferation, aerosol formation, and dispersal must be considered during investigations. In particular, Circox systems and wastewater treatment plants with surface aeration are among these high-risk facilities. (R.A.)