Between July 31 and August 5, 2003, 12 cases of Legionnaires' disease caused by Legionella pneumophila serogroup 1 (Lp1) were reported to the Hérault Departmental Directorate of Health and Social Affairs (DDASS). An epidemiological and environmental investigation was initiated by the DDASS and the Southern Interregional Epidemiology Unit (CIRE), in collaboration with public health surveillance partners. The objectives were to describe the outbreak, identify the common source(s) of contamination, and propose control measures. (R.A.)