Three indigenous groundwater bacterial strains and Pseudomonas putida harboring plasmids TOL (pWWO) and RK2 were introduced into experimentally contaminated groundwater aquifer microcosms. Maintenance of the introduced genotypes was measured over time by colony hybridization with gene probes of various specificity. On the basis of the results of colony hybridization quantitation of the introduced organisms and genes, all introduced genotypes were stably maintained at approximately 10(5) positive hybrid colonies g-1 of aquifer microcosm material throughout an 8-week incubation period. Concomitant removal of the environmental contaminants, viz., toluene, chlorobenzene, and styrene, in both natural (uninoculated) and inoculated aquifer microcosms was also demonstrated. The results indicate that introduced catabolic plasmids, as well as indigenous organisms, can be stably maintained in groundwater aquifer material without specific selective pressure for the introduced genotypes. These results have positive implications for in situ treatment and biodegradation in contaminated aerobic groundwater aquifers.