An experimental approach based on the assembly of genes of a catabolic pathway was used to detect transconjugants in aquatic communities. Resistance to phenylmercury acetate was established in transconjugants when wide-host-range conjugal plasmids containing merB, the gene encoding organomercurial lyase, were transferred to strains from aquatic communities that had been acclimated to inorganic mercury. These communities were enriched for populations containing merA, the gene encoding mercuric reductase. Conjugation was confirmed, using the plasmids' encoded antibiotic resistance patterns and by hybridization with a eukaryotic gene. Three merB-conjugal plasmids, belonging to incompatibility groups W (pGTE16), P1 (pGTE26), and N (pGTE25) were prepared. Transfers by filter matings of pGTE16 and pGTE26 from Pseudomonas aeruginosa PA01 to indigenous strains, were at efficiencies of .045 and .0048 transconjugants/potential recipient (inorganic mercury resistant strains), respectively. These efficiencies were from one or two orders of magnitude below those observed for intraspecies matings with single strains. The third plasmid, pGTE25, was not stably maintained in P. aeruginosa donors and its transfer from Escherichia coli donors was below the level of detection. Characterized transconjugant strains were shown to be Pseudomonas spp. Thus, recipient bacteria with the ability to expand their mercury resistance patterns to organomercurials are abundance in aquatic communities. The utility of the experimental approach to studies in microbial ecology and to environmental management is discussed.