The fate of Aeromonas hydrophila, Alcaligenes denitrificans, Vibrio cholerae non-01, Pseudomonas putida and four different isolates of Escherichia coli in fresh river water were assessed by using different microcosms (i.e., membrane diffusion chamber and Erlenmeyer flask). When water samples were incubated at 16 +/- 1 degrees C, the differences in extent of survival among test bacteria were in general not significant. If the incubation temperature was raised to 29 +/- 1 degrees C, in the in situ studies, none of the added bacterial population could be detected by Day 3. In the in vitro studies, two of the four E. coli tested remained detectable by Day 3. Similarly, populations of the introduced A. hydrophila, P. putida and A. denitrificans were still detectable by Day 5. In general, all test bacteria survived better under low incubation temperature, regardless of whether the experiments were carried out under in vitro or in situ conditions. The results clearly indicated that when studying the fate of the introduced bacteria in the aquatic environment, in situ study was definitely required, especially in the summer time.