Toxicity and sorption of five azo and triphenylmethane dyes to freshwater microbiota were determined to assessment, in part, the risks that these dyes may pose to the aquatic environment. The toxicities of Basic Violet 1, Basic Violet 2, Basic Violet 3, Basic Green 4 and Tropaeolin 0 were established by determining the percent survival of microbiota when dyes were incorporated into plating media. Basic Violet 3 was the most toxic, with a mean survival rate of 20.7 + or - 6.5% at a dye concentration of 5.0 mg/l. Tropaeolin 0 was the least toxic, with a survival rate of 92.0 + or - 9.5%. Survival increased with decreasing dye concentration. Sorption of the dyes to viable cells required from 8 to 24 h to reach equilibrium. Partition coefficients were higher for viable cells than for heat-killed cells, suggesting that a metabolic process may be involved in sorption of these dyes or that autoclaving the cells reduces the organisms' cation exchange capacities.