Two second-order transformation rate coefficients--kb, based on total plate counts, and kA, based on periphyton-colonized surface areas--were used to compare xenobiotic chemical transformation by laboratory-developed (microcosm) and by field-collected microbiota. Similarity of transformer community structure in blended aufwuchs was indicated by kb values, and similarity of transformation rates per unit of periphyton-colonized surface area was indicated by kA values. Xenobiotic chemicals used for the comparisons were methyl marathion (MP), diethyl phthalate (DEP), and 2, 4-dichlorophenoxyacetic acid butoxyethyl ester (2, 4-DBE). Even though rate coefficients of each of the chemicals were similar among microcosm- and field-collected microbial samples, not all of the field collected samples showed MP or DEP transformation. MP transformation was suppressed by aqueous extracts of field-collected, algae-dominated aufwuchs and by low concentrations of DEP. Lack of DEP transformation appeared to have resulted from an absence of DEP-transforming bacteria in field-collected aufwuchs samples.