Accurate water quality criteria are essential if fisheries are to be protected without unnecessary restrictions being imposed on the discharge of industrial and domestic effluents. There is a considerable literature on the effects of pollutants on fish, but even for the common poisons it is difficult to establish more than tentative criteria from the published data. Acute toxicity tests can be used to measure the effect of chemical and physical variables on the toxicity of poisons and on the resistance of fish to them. Results from sublethal tests can give an insight into the mechanism of toxic action and experiments should be designed to show the level of no adverse effect. Field observations can provide valuable information on the levels of pollutants at which fisheries are unaffected and, in some cases, the graded effect of increased pollution on the deterioration of a fish population. Information from all three sources are required for the preparation of water quality criteria, which should not be based on a single concentration, or ratio of a lethal level, but should indicate, where possible, the range of concentration within which fisheries show a progressive deterioration.