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 K-ow) 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 K-ow, and it was concluded that high K-ow 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.