Genetic exchange between bacteria in natural habitats is believed to be strongly influenced by availability of growth substrates and growth rate. To test this hypothesis, conjugal transfer of an RP4 derivative between Pseudomonas fluorescens and Serratia sp. was studied in a simple sand/plant microcosm and was related to availability of root exudates and bacterial metabolic activity. The presence of the plant (Echinochlora crusgalli) greatly stimulated transfer. Average transfer ratios (T/D·R) were 8.9 x 10^{- 11} in the rhizosphere and 5.5 x 10^-15 in sand unaffected by the plant root. The bacteria used root exudates as growth substrate and depending on cell density, the metabolic activity ([³H]leucine uptake) of the cells was higher in the rhizosphere than in the sand (around 1.0 x 10^-2 and 0.5 x 10^-2 fmol leucine CFU^-1 h^-1, respectively). Thus, an apparent correlation between metabolic activity and transfer was observed. Additional experiments, however, suggested that there was no causal relationship between the two. When incubated in a sand microcosm containing root exudates, the metabolic activity of the bacteria increased while conjugal transfer ratios remained constant. Hence, contrary to the accepted view, root exudates and metabolic activity did not appear to be responsible for the stimulation of conjugal transfer in the rhizosphere.