The accumulation of metabolites in the aqueous phase was measured during studies of m-cresol biodegradation with natural water samples. 14C-m-cresol was separated from radiolabeled products remaining in the aqueous filtrates using HPLC. In heterotrophic activity studies, which involve short-term incubations (< 24 h), metabolite accumulation accounted for only a small fraction of the radiolabeled m-cresol originally added. After longer incubations characteristic of die-away studies, metabolite accumulation accounted for as much as 30% of the label originally added. The nature of metabolite accumulation varied, and appeared to reflect differences in the initial environmental samples. Sometimes no metabolite accumulation was detected. One time, a few products accumulated and they were less polar than m-cresol. At other times, many different products accumulated and they were more polar than m-cresol. When metabolite accumulation occurred, it was greater in samples receiving 960 ug/1 m-cresol than 120 ug/1 m-cresol. No one metabolite accumulated consistantly to a level greater than 5% of the radiolabel originally injected.