Ten of twelve men who participated in the cleaning of an electric power steam turbine condenser clogged with freshwater sponge experienced an acute febrile illness. Two similar outbreaks have been previously described, one of which has been attributed to the Legionnaires' Disease bacterium. Epidemiologic studies of this case showed a syndrome very similar to the two previously reported episodes. However, the exact etiology for this outbreak has not been identified. Environmental sampling was not initiated until after the cleaning was completed. Subsequent testing did not reveal any likely cause for the outbreak. The delayed onset of symptoms and the nature of the illness pointed to an infectious agent. In the absence of any suitable criteria for employee exposure evaluation, it is suggested that crews cleaning condensers under unusually dirty conditions, especially if eye or respiratory symptoms are reported, should be provided with respiratory protection.