Radioactive, inorganic arsenic was administered to brown trout (Salmo trutta) orally and by intramuscular injection. The orally administered arsenic appeared in the tissues in an altered form that is presumed organic; this organoarsenical was also found in the gastrointestinal contents. The injected arsenic appeared in the tissues in inorganic form and was only slowly converted to organic form; meanwhile, large amounts of inorganic and organic arsenic were detected in the bile. These findings are consistent with biosynthesis of the organic arsenic compound within the gastrointestinal tract.