Samples of the microbial community from Lake Michie, a mesotrophic reservoir in central North Carolina, were adapted to various levels (100 to 1,000 micrograms/liter) of natural humic acids in chemostats. Adaptation to increasing levels of humic acids significantly reduced the ability of the microbial communities to degrade all three phenols. Short-term exposure of the community to increasing levels of humic acids had no significant effect on the ability of the community to degrade m-cresol. Thus the suppressive effect of humic acids on monosubstituted phenol metabolism was the result of long-term exposure to the humic materials.