Cell survival and plasmid stability in Escherichia coli containing plasmids RP1, R388 and pUB824 were studied in raw waste water, treated waste water, and brackish water. The Escherichia coli strain survived well in raw and treated waste water. However, in brackish water E. coli HB101 remained culturable throughout the microcosm study, with an overall drop of 2 log units in culturability from the start to the end of the experiment. The maintenance of the three plasmids was plasmid-, and environment-dependent. Plasmid pUB824 (4 kb) was stably maintained under all conditions used in the study. Maintenance of RP1 (56 kb) and R388 (33 kb) was markedly influenced by nutritive conditions, which caused a segregation of the plasmids from cells. The results of the present study demonstrate the influence of plasmid size on plasmid stability in natural waters.