The incidence and density of Legionella spp. in raw water, water at various stages of treatment, and in potable distribution water were determined by direct immunofluorescence. The number of cells reacting with Legionella-specific fluorescent antibody conjugates in raw waters ranged from about 10,000 to 100,000 cells/liter, whereas the concentrations of fluorescent antibody-positive cells in the distribution waters were generally 10- to 100-fold lower than in the raw source waters. No viable or virulent Legionella strains were isolated from either the source or distribution waters. However, Legionella sp. are infrequently isolated from water at temperatures below 15C as was the case in the systems surveyed in this study.