Between June 1981 and December 1982 the incidence of Vibrio cholerae, V. mimicus and V. parahaemolyticus was determined at two sampling sites on the Elbe River at Hamburg. A total of 183 strains was isolated from 147 water samples. Of these, 107 belonged to non-01 V. cholerae (ten strains producing a cholera-like enterotoxin); 33 were identified as V. mimicus, including two enterotoxin producers; 42 strains were Kanagawa-negative cultures of V. parahaemolyticus; and one was V. fluvialis. The highest incidence was observed from June to September with about 10(2) organisms/l. Halophilic vibrios, less than five organisms/l, were detectable during the period June/July to October. The vibrio incidence was not influenced by the numbers of aerobic heterotrophic bacteria, coliforms or faecal bacteria. In general water temperature correlated with the seasonal variation. Thus, a temperature rise over 10 degrees to 20 degrees C was followed by a distinct increase in vibrio numbers. Of 14 chemical parameters only chloride concentration might have had an influence on the seasonal variation. It is concluded that the three Vibrio species are indigenous organisms of the Elbe River.