The acute toxicity (48-hr LC50) of lindane (gamma-HCH) to 16 fish species, belonging to eight families, ranges from 22 to 900 micrograms/liter (mean: 150 micrograms/liter). A significant positive linear relationship between the lipid content (% on a wet weight basis) of the fishes and their toxicity to gamma-HCH was found. If the toxicity is referred to 1% lipid, 48-hr LC50 values range between 13.2 and 32 micrograms/liter, and thus the coefficient of variation of the mean is reduced from 139 to 22%. It is concluded that the lipids of aquatic organisms serve as a protective reservoir against the toxic effects of lindane and other lipophilic, relatively persistent organic chemicals, because they are bioconcentrated mainly in the body lipids. Therefore, in organisms with high lipid content, only a relatively small fraction of the hydrophobic chemical can reach target organs (nerves, liver, etc.) and/or receptors. For comparing toxicity data of organic chemicals to aquatic organisms, the total lipid content of the organisms must be considered. The results of this investigation are important in comparative environmental toxicology for risk assessment of freshwater and marine organisms.