The effect of concentration on the biodegradation of synthetic organic chemicals by natural microbial communities was investigated by adding individual C-labeled organic compounds to stream water at various initial concentrations and measuring the formation of CO(2). The rate of degradation of p-chlorobenzoate and chloroacetate at initial concentrations of 47 pg/ml to 47 mug/ml fell markedly with lower initial concentrations, although half or more of the compound was converted to CO(2) in 8 days or less. On the other hand, little mineralization of 2,4-dichlorophenoxyacetate and 1-naphthyl-N-methylcarbamate, or the naphthol formed from the latter, occurred when these compounds were present at initial concentrations of 2 to 3 ng/ml or less, although 60% or more of the chemical initially present at higher concentrations was converted to CO(2) in 6 days. It is concluded that laboratory tests of biodegradation involving chemical concentrations greater than those in nature may not correctly assess the rate of biodegradation in natural ecosystems and that low substrate concentration may be important in limiting biodegradation in natural waters.