Species of Candida, Gliocladium and Penicillium from polluted waters were found to produce trimethylarsine gas from several arsenic sources. The pH of the solution and phosphate ions had a marked effect on the biosynthesis of this volatile arsenic compound. A strain of Penicillium which produced dimethylselenide from inorganic selenium compounds was isolated from raw sewage. Sulfate and methionine enhanced growth of the fungus. In solutions containing selenate, methionine inhibited dimethylselenide formation while stimulating proliferation of the fungus. Dimethylselenide was also generated from inorganic selinide. Dimethyltelluride was also produced by the organism from several tellurium compounds. The significance of these processes in natural water and the potential hazards to man or aquatic life remain to be assessed.