Transformation rates of an insecticide, methyl parathion, in pure cultures of Flavobacterium sp. followed multiphasic kinetics involving at least two systems (I and II). System I was a high-affinity, low-capacity system, and System II was a low-affinity, high capacity system. Data from rate experiments suggested that metabolites formed via system II inhibited system I such that only one system operated at a time. System I operated at approximately 20 micrograms per liter and less; system II operated at approximately 4 mg per liter and less. These results show that xenobiotic chemicals, like naturally occurring substrates, can be transformed via multiple uptake and transformation systems even by a pure culture. Furthermore, computer simulation models of pollutant transformation rates based on kinetic constants determined in this study show that large errors can occur in predicted rates when the multiphasicity of kinetics is neglected.