Pathogenic microorganisms were studied in river waters, ground water and recreational hot baths. Free-living amoebae and bacteria were located in their natural sites, and the influence of physical and chemical variations on their ecology was studied. Microbiological contamination increases from ground water to the rivers and depends on human and animal activities. Rhine waters are heavily contaminated with Enterobacteriaceae, pathogenic Naegleria and Legionella. Amoebae are likely to be natural hosts for Legionella. Intra-cellular replication and inclusion in resistant cysts of amoebae give them a possibility of survival in unfavorable conditions. The heavy amoebical and bacteriological contamination of recreational hot baths poses a potential risk for swimmers. Cleaning and chlor-ination of the pools is a good preventive against waterborne infections.