The overwhelming number of industrial chemicals either in commerce or in development precludes actual toxicity testing of each chemical and creates a need for initial screening techniques to focus efforts on those chemicals with the greatest potential for adverse environmental effects. One promising screening technique is the use of the structure-activity relationship to estimate aquatic toxicity from chemical structure. Using five major classes of industrial chemicals from the Toxic Substances Control Act (TSCA) inventory, this paper presents a structure-activity relationship between the 96-h LC50 with fathead minnows and the n-octanol/water partition coefficient which can be computed from structure. Common industrial alcohols, ketones, alkyl halides, ethers, and benzenes are shown to cause lethality by the same mode of action of narcosis. Consequently, a single bilinear equation can predict adequately the 96-h LC50 of all classes of industrial chemicals to fathead minnows.