During the bathing period of 1981 (June to September), and before it (September 1980 to May 1981), we monthly collected, and analysed, samples of water from a series of lakes used as recreational bathing area by inhabitants of the southern part of the City of Bratislava. We isolated and determined biochemical properties of altogether 241 strains of gram negative bacteria belonging mostly to Enterobacteriaceae and to so-called non-fermenters, with special reference to deviations in results of individual tests used for their taxonomical identification and classification. We attempted to determine also the significance of individual biochemical tests of bacterial strains from water samples, as well as the general evaluation of water quality on basis of hygienic criteria set up for the suitability of individual lakes for recreational bathing, to analyse the causes of water contamination, and to evaluate bacterial strains from the hygienic and epidemiological point of view. The qualitative and quantitative incidence of individual bacterial biotypes indicate a continuous contamination of natural baths with fecal waste containing microorganisms from healthy as well as sick persons. As a consequence, the free bathing of the City population in natural lakes studied represents a potential sanitary risk for the persons concerned.