The acute toxicity of Sevin® (1‐naphthyl N‐methylcarbamate) and its hydrolytic product 1‐naphthol to 10 species of marine animals was determined. Sevin was more toxic to larval and adult crustaceans (Upogebia pugettensis, Callianassa californiensis, Hemigrapsus oregonensis, Cancer magister) than to larval and adult mollusks (Mytilus edulis, Crassostrea gigas, Clinocardium nuttallii) and juvenile fishes (Cymatogaster aggregata, Parophrys vetulus, Gasterosteus aculeatus). Sevin was 30 to 300 times more toxic than 1‐naphthol to the crustaceans but less toxic than 1‐naphthol to the mollusks and fishes.