Using batch cultures, the authors determined transformation rates for low concentrations of two toxicants--an insectide, methyl parathion, and a plasticizer, diethyl phthalate--by aufwuchs. Aufwuchs samples were collected from field sites, an indoor channel, and a continuous-flow fermentor. Aufwuchs fungi, protozoa, and algae did not tranform the chemicals but bacteria rapidly transformed both. Second-order transformation rate coefficients, Kb, based on total plate counts of bacteria in aufwuchs, were determined for potential use in a mathematical model capable of predicting the transport and fate of chemicals in aquatic systems. Kb for both chemicals decreased as the concentration of total bacteria, (B), increased in aufwuchs. This effect resulted from the proportion of nontransformer to transformer bacteria increasing as (B) increased and from the rate of transformation per transformer cell decreasing as (B) increased. First-order transformation rate coefficients, K1, were relatively stable per unit of surface area colonized by aufwuchs because Kb decreased as (B) increased (K1 = Kb X (B)).