Laboratory experiments are described in which guppies (Poecilia reticulata) were fed food containing hexachlorobenzene, mirex, 2,2′,3,3′,4,4′,5,6′-octachlorobiphenyl, and octachlorodibenzofuran. The chemical concentrations in the food and the feeding rate were varied to determine the effect of these variables on the assimilation efficiency. Increasing the dietary intake by increasing the concentration (or fugacity) of a chemical in the food causes a proportional increase in the steady-state concentration or fugacity of the chemical in the fish; i.e., the assimilation efficiency is constant. Increasing the dietary intake by increasing the feeding rate produces a less than proportional increase in the steady-state concentration or fugacity of the chemical in the fish. This apparent drop in assimilation efficiency is the result of an increased rate of loss by egestion. Mirex and octachlorobiphenyl were observed to biomagnify; i.e., the steady-state fugacities achieved in the fish exceeded the fugacities in the food. These findings are in accord with the assertions of a fugacity model of bioaccumulation.