A microcosm is described to evaluate and measure bacterial conjugation in the rhizosphere of barley andd radish with strains of Pseudomonas cepacia. The purpose was to describe a standard method useful for evaluating the propensity of genetically engineered microorganisms (GEMs) to transfer DNA to recipient bacteria. Results demonstrated the formation of transoon jugants from the rhizosphere of each plant 24 h after inoculation. Transconjugant populations peaked at 180 colony forming units (CFU)/g root and associated soil in barley and 200 CFU/g root and associated soil in radish; they then declined over the next five days of the experiment. No significant differences were found in the survival of transconjugant populations monitored from the two plant species. The microcsm was also used to document the formation of false positive transcojugants, which resulted from donor and recipient P. cepacia mating on the surface of selective agar plates instead of in microcosms. Transconjugants resulting from such plate mating occurred in substantial numbers during the first 5 days of the experiment but declined to undetectable numbers by day 7. The use of nalidixic acid was investigated to determine the magnitude of plate mating. The number of transconjugants detected from radish rhizosphere was reduced by two orders of magnitude by including nalidixic acid in the plating medium; this indicated that 99% of the transconjugants were a result of plate mating. (Copyright (c) 1991 Springer-Verlag New York Inc. 1991.)