Biomagnification of HCB and PCBs was measured in a natural fish population of white bass (Morone chrysops) and their prey, emerald shiner (Notropis atherinoides). It was observed that biomagnification occurred only for chemicals with an octanol/water partition coefficient (log Kow) greater than 6.1. Hexachlorobenzene and PCB 52 did not biomagnify while PCBs 87, 138, and 180 showed significant biomagnification in Lake Erie white bass. Biomagnification factors increased proportionately with Kow, and it was concluded that high Kow chemicals were more important in the biomagnification process. Lipid proportions and chemical concentrations in prey fish (emerald shiner) and white bass intestinal contents were consistent with a fugacity model of chemical assimilation from food.