The survival times of Salmonella enteritidis, Pseudomonas aeruginosa, Staphylococcus aureus, and Vibrio cholerae in natural soil and river water samples were monitored by using various differential media. S. aureus and V. cholerae failed to survive in samples of soil, groundwater, and river water with various degrees of eutrophication. The population of the introduced S. enteritidis remained fairly constant in all three samples of soils tested. In water samples, numbers of S. enteritidis and P. aeruginosa showed an initial rapid decline followed by a much lower rate of decrease. The results indicated that some allochthonous microorganisms, because of their insensitivity to various biotic and abiotic stresses, might persist for a long time in the environment and become a serious threat to public health.