Although Legionella spp. are often isolated from natural aquatic habitats, outbreaks of legionellosis are rarely traced to these sources. To determine the fate of Legionella pneumophila in the environment, filtered and unfiltered river water and seawater microcosms, incubated at 4 degrees C and 26 degrees C, were inoculated with [3H]thymidine-labeled L. pneumophila cells. Survival in these microcosms was monitored using [3H]thymidine labeling and culture on buffered-charcoal yeast extract agar amended with alpha-ketoglutarate (BCYE alpha). Immunofluorescent microscopy, direct fluorescent antibody staining, and acridine orange direct counts were also employed. To assess effects of grazing on Legionella, a duplicate set of samples was filtered through 2.0-microns Nuclepore filters to trap large protozoa. Over the test period, in the microcosms incubated at 4 degrees C, the culturable counts decreased ca. 1 log on BCYE alpha agar, with no substantial decline in thymidine count. Autoclaved seawater and river water controls held at 15 degrees C also showed no change in thymidine count. At 26 degrees C, a 3-log decline was observed in culturable counts, with ca. 1-log decline in thymidine counts. These results indicate that, although culturability declined by one to three orders of magnitude, when L. pneumophila microcosms were incubated at 4 degrees C and 26 degrees C, the cells remained metabolically active for extended periods, especially at 4 degrees C.