The microbial degradation of a number of 14C-labeled, recalcitrant,aromatic pollutants, including trichloroguaiacol and di-, tri-,and pentachlorophenol, was investigated in aquatic model systemsin the laboratory. Natural, mixed cultures of microorganismsin the water from a brown-water lake with a high content ofhumic compounds mineralized all of the tested substances toa higher degree than did microorganisms in the water from aclear-water lake. Dichlorophenol was the most rapidly degradedpollutant.