The InVS regional office in the Pays de la Loire investigated an outbreak of Q fever that occurred in Maine-et-Loire in early 2009 at a facility comprising a cattle slaughterhouse and a meat processing plant employing 1,000 workers. The investigation focused on actively identifying cases within the facility from January through March 2009 and on identifying potential sources of contamination and modes of transmission. During the outbreak period in February 2009, 50 cases were confirmed by the presence of Coxiella burnetii-specific IgM. The epidemic curve followed a log-normal pattern, suggesting a single-point or very time-limited exposure. The probable period of contamination occurred during the second half of January. The attack rate of confirmed cases among the company’s employees was 5.4%. The highest attack rates were observed in the meat processing workshops (8–10%) and the packaging preparation workshops (41%). No cases were observed in the slaughterhouses. Several potential sources of contamination were examined. Exposure to fetuses or placentas, known to carry a very high bacterial load when infected, was considered the most likely cause. Thus, the rupture of a bag containing fetal calf blood in a transport vehicle parked in the factory’s clean yard on January 29, 2009, was deemed the most probable source. The mode of transmission, usually via aerosols, remains poorly understood; the mist generated by pressure washing of potentially contaminated floors or equipment could explain the distribution of cases in workshops that were, in principle, not highly exposed to the risk of Q fever. (R.A.)