Microorganisms in soils and waters convert many synthetic organic chemicals to inorganic products. Other compounds are transformed only by cometabolism. These microbial processes may lead to environmental detoxication, the formation of new toxicants, or the biosynthesis of persistent products. Type reactions are proposed for major categories of enzymatic transformation of synthetic chemicals in soils, natural waters, and sewage. Some organic molecules are resistant to microbial attack, and explanations for the persistence of such compounds are suggested.