In situ transfer of a self-transmissible, antibiotic-multiresistant plasmid RP4 from a laboratory Escherichia coli strain C600 to indigenous activated sludge bacteria was investigated using filter mating. The transfer frequency of RP4 from the donor E. coli to the bacteria that was sampled from two wastewater treatment plants was 5.1x10(-2) to 7.5x10(-1) and 4.6x10(-3) to 1.3x10(-2)/potential recipient. The isolated transconjugants showed resistance to Ap, Km, and Tc and the presence of a plasmid with a similar size to RP4. The traG gene on RP4 was also detected from all transconjugants. Reverse-transfer experiments from the transconjugants to E. coli HB101 indicated that RP4 maintained self-transmissibility in the transconjugants. The transconjugant strains were dominant bacteria in activated sludge including Pseudomonas fluorescens, P. putida, and Ochrobactrum anthropi and minor populations of enteric bacterial strains including Citrobacter freundii, E. coli, Enterobacter cloacae, E. asburiae, and Klebsiella pneumoniae ssp. pneumoniae. The transconjugant strains K. pneumoniae ssp. pneumonia, E. cloacae, and E. asburiae had several naturally occurring plasmids. These results suggest that in situ transfer of plasmids and the exchange of antibiotic-resistant genes can occur between released and indigenous bacteria in activated sludge.