A survey for legionella in public water supplies in England failed to detect culturable L.pneumophila in all but samples from taps in buildings; however, the organism was detected in underground and surface water sources and distribution systems using an indirect immunofluorescent assay (IFA) to L.pneumophila serogroup 1. Culturability was related to water temperatures above 20°C. In water mains L.pneumophila was associated with biofilms or sediment. Non-culturable L.pneumophila detected in potable waters by IFA were recovered by heat shock experiments demonstrating their viability. Although the strain found in potable water is rarely asociated with disease, monoclonal expression, a virulence marker, was altered by heat shock. These findings have implications for the prevention of legionellosis and may explain the sporadic nature of legionnaires disease in the community.