An outbreak of Legionnaires' disease affected 12 customers of a supermarket in a town in Catalonia, Spain, between August and November 2006. An epidemiological and environmental investigation was undertaken. Preliminary investigation showed that all patients had visited the same supermarket in this town where a mist machine was found in the fish section. Water samples were collected from the machine and from the supermarket's water distribution system when high-risk samples were excluded. Environmental samples from the mist machine and clinical samples from two patients tested positive for L. pneumophila serogroup 1 and had the same molecular pattern. The PFGE pattern detected in the clinical and mist-machine isolates had never previously been identified in Catalonia prior to the outbreak and has not been identified since. Four days after turning off the machine, new cases ceased appearing. Molecular study supports the hypothesis that the mist machine from the fish section of the supermarket was the source of infection. We believe it is essential to include exposure to mist machines in any legionellosis epidemiological survey.