Three outdoor ponds were treated with methyl parathion (MEP) applied beneath the water surface at a concentration of 100 micrograms liter-1. Laboratory aquaria containing either tap water, pond water, tap water plus plants, tap water plus sediment, or tap water plus sediment and plants were similarly treated. Samples of water, sediment, and fish were analyzed for residues of MEP. The rate of loss from water and concentrations found in sediment were compared with predictions based on a calculated rate of biodegradation and a sediment:water partition coefficient. The rate of loss of MEP from pond water isolated in an aquarium was similar to the predicted rate. However, the rate of loss from outdoor ponds, or from aquaria containing plants and sediment, was greater than predicted. MEP was not detected in sediment even though predicted concentrations far exceeded the limit of detection. These results are discussed and it is suggested that the rate of biodegradation in shallow bodies of water may be determined predominantly by bacteria attached to sediments and plants, rather than by planktonic bacteria. Bioaccumulation in fish was predicted from empirical equations based on the octanol:water partition coefficient. Observed values were in good agreement with predictions.