It has been suggested that the herbicide atrazine may cause increased incidence of limb deformities in amphibians by inhibiting growth of phytoplankton and thereby indirectly stimulating growth of periphyton, populations of aquatic snails and their hosted trematodes. I examined the impacts of environmentally relevant concentrations of atrazine on phytoplankton, periphyton and snails, and tested the influence of nutrients on atrazine toxicity. In two outdoor microcosm studies, concentrations of atrazine up to 200 [mu]g/L generally did not affect biomass of phytoplankton or periphyton, or populations of aquatic snails. When concentrations of total phosphorus were < 2001 [mu]g/L, atrazine significantly affected DO, pH, and conductivity, and additions of phosphorus helped to mitigate this effect in a concentration-dependent manner. These observations indicate that the proposed indirect mechanism leading to limb deformities in amphibians does not proceed under all environmental conditions, even at unrealistically high concentrations of atrazine and particularly when nutrients are non-limiting.