AbstractTerms for decomposition by both suspended and attached microorganisms were included in a model to calculate the fate of chemicals in aquatic systems. This model differs from previous models primarily by the inclusion of a term for decomposition by aufwuchs in addition to a term for decomposition by planktonic microorganisms. Decomposition by aufwuchs is represented as occurring at a rate limited by the serial processes of transport to a planar surface and decomposition at the surface. Two sets of experiments were conducted using replicate laboratory microcosms, each maintained with six different environments. The predictive capability was good, as indicated by the absence of statistically detectable differences between predictions and laboratory measurements. In making the predictions, a set of parameters describing the characteristics of the chemical was required. Included in these parameters were the rate coefficients for the two microbial decomposition terms. With both microbial decomposition terms included, the fate model accounted satisfactorily for observed methyl parathion concentrations using only a single, environment‐independent rate coefficient for each ter