This report briefly synthesizes technical literature on ecological and toxicological aspects of arsenic in the environment, with special reference to natural resources. It is divided into a number of subtopics: sources, fate, and uses; chemical and biochemical properties; essentiality, synergism, and antagonism; background concentrations in biological and nonbiological samples; lethal and sublethal effects, including carcinogenesis, tertogenesis, and mutagenesis; and proposed criteria and research needs for the protection of sensitive organisms. parag Keywords: contaminants; toxicity; fishes; wildlife; natural resources; invertebrates; arsenic; organoarsenicals; carcinogenesis; teratogenesis; mutagenesis; metabolism.