Simple assumptions for individual toxic response, exchange of toxicant with environmental concentrations and body composition are used in a model to evaluate the effect of lipid variation on toxic response in a subpopulation of similarly sized individuals. This model represents the internal distribution of a chemical such that more hydrophobic chemicals preferentially move into body lipid. Thus, for exposures of equal chemical activity, both increasing body fat and greater hydrophobicities increase the exposure duration that can be withstood without effect. In simulated 96 h bioassays the effect of increased tolerance to higher hydrophobicities was apparent for chemicals whose KOW exceeded 104. These simulations are compared to published observations. Simulations are also compared to other published data for longer-term bioassays. The effect of interspecies gill morphology on toxic response is also explored. It is concluded that variation in lipid can account for much variation in tolerance in a subpopulation of similarly sized individuals; that gill morphology is another variable influencing toxic response; and that, in general, for similarly exposed organisms, the fattest survives the longest.