Seven antibiotic-multiresistant Escherichia coli strains, possessing three or four plasmids, capable of transferring their resistance marker at a high frequency, were selected among a total of 300 antibiotic-resistant E. coli strains isolated from natural water-raw and treated wastewater, and brackish water (collected 1 km downstream). These strains were mated with E. coli K-12 C600 nal(r), both in sterilized natural water and LB medium at 25 degrees C. Conjugation did occur in all. the systems tested, although fewer transconjugants were recovered from raw and treated wastewater experiments. In contrast, in brackish and seawater, the transfer frequency did not significantly decrease in spite of salt contents. In 100% of the cases, transfer of the high-molecular-weight plasmids (greater than or equal to 20 kb) was observed, but the small plasmids (2.6-7.5 kb) were only cotransferred in raw or treated wastewater and in brackish water. Moreover, genotypic variation occurred more frequently in natural water than in LB medium.