Chemicals designated as "priority pollutants" or "toxics" have received special attention recently because the discharge of these compounds into public water is to be restricted to the maximum possible with little regard to water quality or economics. The selection of many of the 129 priority cemicals was not based on an objective scientific assessment of the exposure and effect data. In fact, for some compounds, including acenaphthene and 4-chlorophenyl-phenyl ether, the necessary data for listing were non-existent. As an alternative to arbitrarily listing or delisting chemicals for the purpose of prioity control, this paper suggests a promising scientific approach to selecting priority chemicals based on the principles of hazard assessment for chemicals in the aquatic environment. According o the hypothesis, the highest priority chemicals are those with the least margin of safety, defined as the gap between the no-observable-effect concentrations and the ambient exposure concentrations. The no-observable-effect concenrations are based on the results of chronic or sensitive life stage tests with aquatic organisms and the acceptable daily intake rate for fish eates. The ambient exposure concentrations are levels either measured in fish and water, or roughly estimated from a simple nomogram that requires only two of the following three factors: environmental release rate, ratio of dissipation to bioconcentration potential, or ambient residues in fish. THE CHEMICALS STUDIED TO ILLUSTRATE THIS APPROACH TO PRIORITIZING CHEMICALS BASED ON HAZARD ASSESSMENT ARE: polychlorinated biphenyls, di-2-ethylhexyl phthalate, linear alkylbenzene sulfonate, and pentachlorophenol.