To add the knowledge on the plasmid-mediated bioaugmentation in the activated sludge process, conjugative transfer of a self-transmissible, broad host range, mercury resistance, and partial 2,4-dichlorophenoxyactic acid (2,4-D)-degrading plasmid pJP4 from Escherichia con HB101 or Pseudomonas putida KT2440 to activated sludge bacteria were examined by filter mating. Activated sludge samples were collected from an actual wastewater treatment plant and laboratory reactors operated under different sludge retention times (SRTs) of 5, 10, and 15 days. Transfer of OM was observed in all combinations of donor and activated sludge (recipient) with the transfer frequency from 6.5 x 10(-8) to 2.5 x 10(-4) transconjugants per recipient, except from E. coli HB101 to the activated sludge sample of SRT 5 days. Although plasmid pJP4 was transferred mainly to dominant bacteria belonging to Pseudomonas, the host range of pJP4 in the activated sludge varied depending on the donor strain. Phenotypically various transconjugants were isolated from activated sludge sampled from the reactors with longer SRT operations. All transconjugants exhibited high mercury resistance equivalent to the donor strains; some were capable of degrading 2,4-D. These results suggest that the behavior of plasmids can be controlled partially through selection of appropriate donor strains and operational conditions of the activated sludge process. (C) 2010, The Society for Biotechnology, Japan. All rights reserved.