An experiment was conducted to investigate the variability in biotransformation kinetics of xenobiotics in natural waters by aufwuchs communities. Two chemicals were used, 2,4-di-chlorophenoxyacetic acid butoxyethyl ester (2,4-DBE) and para-cresol. Teflon strips were colonized at two sites each in two ponds and two rivers. The colonized strips and the natural waters were transported to the laboratory where the biotransformation rate studies were done in duplicate under controlled conditions. Analysis of variance at the 0.10 level of significance revealed a significant difference in biotransformation rate coefficients between ponds and between sites within one pond for both xenobiotics, and between rivers for para-cresol only. No significant difference was found between sites within rivers for either xenobiotic, nor was any significant difference found between rivers for 2,4-DBE. In this study, differences in biotransformation rates were less variable and more detectable in quiescent waters than in faster flowing waters. (Copyright (c) Pergamon Journals Ltd. 1987).