The purpose of these studies was to investigate a variety of ecologically significant interactions between dimethyl mercury and the aquatic environment in which it may be present. Laboratory studies indicate dimethyl mercury may be a major product of microbial methylation in organic mercury. The physical, chemical, and biological factors affecting the transport and food chain distribution of dimethyl mercury have remained unclear. Results are presented of laboratory studies of volatilization rates of dimethyl mercury from water as a function of temperature and mixing conditions. Absorption kinetics and equilibrium concentrations of dimethyl mercury in algae, Daphnia, and fish are reported. Toxicity to fish, and studies of metabolism of the compounds by microbes are discussed. Mercury-203 labeled dimethyl mercury was used in the study, and in all tests the organisms used were live counted. A unique method was developed for measuring dimethyl mercury uptake in algae. In water, dimethyl mercury was found to behave similarly to nonreactive gases such as oxygen.