An experiment was conducted to investigate the variability in biotransformation rate coefficients of a xenobiotic, the butoxyethyl ester of 2,4-D, in natural waters between aufwuchs grown on Teflon strips located on the bottom and at the water surface of a pond and a river. The colonized strips and the natural waters were transported to the laboratory where the biotransformation studies were done under controlled conditions. Statistical analyses applied to the first-order rate coefficients showed a significant difference between bottom and surface aufwuchs for the river only. For both pond and river, a significant difference was shown when the aufwuchs was suspended, however. The aufwuchs mat thickness was significantly different between bottom and surface for the pond but not for the river and the biomass as ashfree dry weight was significantly different for both water bodies. The variability of biomass and first-order rate coefficients was higher with the bottom colonized aufwuchs than with the surface colonized aufwuchs.