AbstractStudies were conducted to examine the adaptation response of aquifer microbial communities to xenobiotic compounds and the influence of chemical preexposure in the laboratory and in situ on adaptation. Adaptation and biodegradation were assessed as mineralization and cellular incorporation of14C‐radiolabeled substrates. For some compounds, such as ethylene dibromide, aniline andm‐nitrophenol, biodegradation and adaptation rates were not influenced by chemical concentration over the range tested. Biodegradation rates increased with concentration forp‐chlorophenol, and a gradient of adaptation and biodegradation responses was observed forp‐nitrophenol up to a threshold concentration. Acclimation to laboratory conditions decreased but did not eliminate the adaptation period top‐nitrophenol. Laboratory adaptation studies and examination of uncontaminated and contaminated field samples from a single aquifer indicated that adaptation was accompanied by shifts in the metabolic fate of the substrate. The increased metabolism associated with adapted communities was predominantly the result of increased mineralization. Cellular incorporation represented a significantly smaller fraction of total metabolism in the adapted versus the unadapted communities. The results indicate that the adaptation response in aquifer solids is due to a complex set of interactions that are influenced by the physiology and growth of the degrading po