Seasonal effects on microbial transformation rates of an herbicide, 2,4-dichlorophenoxyacetic acid butoxyethyl ester (2,4-DBE), in a freshwater stream were investigated using low concentrations (less than 100 micrograms L(-1)) of the herbicide mixed with an inert dye, rhodamine WT, to correct for dilution. Although water temperatures decreased from 22 + or - 0.5 to 8.0 + or - 0.5 degrees C and water velocities decreased from 0.09 + or - 0.01 to 0.03 + or - 0.003 m s(-1), microbial transformation rates, as indicated by pseudo-first-order transformation rate coefficients, increased as much as threefold in the cold months over those in the spring and summer. The increased winter transformation rates were associated with leaf-litter accumulation, which provided additional colonized surface area in the stream. Therefore, the increased colonized surface area more than compensated for the effects of temperature and water velocity (under conditions of mass-transport limitation) that otherwise would have reduced microbial transformation rates in the winter months. (Copyright (c) 1986, Environmental Toxicology and Chemistry.)