The possibilities for low-frequency horizontal transfer of the self-transmissible chlorocatechol degradative genes (clc) from Pseudomonas sp. strain B13 were investigated in activated-sludge microcosms. When the clc genes were transferred into an appropriate recipient bacterium such as Pseudomonas putida F1, a new metabolic pathway for chlorobenzene degradation was formed by complementation which could be selected for by the addition of mono- or 1, 4-dichlorobenzene (CB). Under optimized conditions with direct donor-recipient filter matings, very low transfer frequencies were observed (approximately 3.5 x 10(-8) per donor per 24 h). In contrast, in matings on agar plate surfaces, transconjugants started to appear after 8 to 10 days, and their numbers then increased during prolonged continuous incubation with CB. In activated-sludge microcosms, CB-degrading (CB+) transconjugants of strain F1 which had acquired the clc genes were detected but only when strain B13 cell densities of more than 10(5) CFU/ml could be maintained by the addition of its specific growth substrate, 3-chlorobenzoate (3CBA). The CB+ transconjugants reached final cell densities of between 10(2) and 10(3) CFU/ml. When strain B13 was inoculated separately (without the designated recipient strain F1) into an activated-sludge microcosm, CB+ transconjugants could not be detected. However, in this case a new 3CBA-degrading strain appeared which had acquired the clc genes from strain B13. The effects of selective substrates on the survival and growth of and gene transfer between bacteria degrading aromatic pollutants in a wastewater ecosystem are discussed.