Degradation pathways for the herbicide clomazone in a California rice field soil were characterized via pulse-labeling of anaerobic (flooded) and aerobic (moist) soil microcosms. Clomazone-derived (13)C in the major C pools of a rice ecosystem and soil phospholipid fatty acid (PLFA) profiles were analyzed over time to determine if (1) the compound accumulates in the microbial biomass, (2) it affects temporal microbial population dynamics, and (3) it is either preferentially metabolized or cometabolized. In anaerobic microcosms, the compound was rapidly biotransformed to ring-open clomazone, upon which it persisted in the aqueous phase, whereas aerobic microcosms degraded it slower but a greater percentage was mineralized. Anaerobic biomass decreased after clomazone was added, and aerobic actinomycete abundance differed between treatments and controls. Additionally, PLFA and (13)C PLFA were statistically similar between treatment and controls. Thus, microbial cometabolism is likely to be the dominant degrading mechanism governing clomazone fate in California rice fields.