Air-lift percolator experiments simulated in situ subsurface degradation of 2,4,6-trichlorophenol, 2.3,4,6-tetrachlorophenol and pentachlorophenol, in mixtures and individually, by indigenous microorganisms from a chlorophenol-contaminated aquifer, Inoculation with a chlorophenol(CP)-degrading gram positive isolate from the CP-contaminated groundwater did not significantly increase CP degradation rates. Feed concentrations of up to 55 mg CP l(-1) were degraded. Stable CP degradation was maintained for over 6 months. CP degradation rates up to 54.3 mg l(-1) day(-1) and effluent concentrations below 40 mu g CP l(-1) were achieved. CP were mineralized as shown by CP reduction, dissolved organic carbon removal and release of inorganic chloride.