To determine the potential for bacteriophage-mediated gene transfer in the marine environment, we established transduction systems by using marine phage host isolates. Plasmid pQSR50, which contains transposon Tn5 and encodes kanamycin and streptomycin resistance, was used in plasmid transduction assays. Both marine bacterial isolates and concentrated natural bacterial communities were used as recipients in transduction studies. Transductants were detected by a gene probe complementary to the neomycin phosphotransferase (nptII) gene in Tn5. The transduction frequencies ranged from 1.33 x 10(-7) to 5.13 x 10(-9) transductants/PFU in studies performed with the bacterial isolates. With the mixed bacterial communities, putative transductants were detected in two of the six experiments performed. These putative transductants were confirmed and separated from indigenous antibiotic-resistant bacteria by colony hybridization probed with the nptII probe and by PCR amplification performed with two sets of primers specific for pQSR50. The frequencies of plasmid transduction in the mixed bacterial communities ranged from 1.58 x 10(-8) to 3.7 x 10(-8) transductants/PFU. Estimates of the transduction rate obtained by using a numerical model suggested that up to 1.3 x 10(14) transduction events per year could occur in the Tampa Bay Estuary. The results of this study suggest that transduction could be an important mechanism for horizontal gene transfer in the marine environment.