A food chain bioaccumulation mathematical framework was developed as part of a broader framework for modeling the fate of toxic chemicals in natural water systems, entitled WASTOX. A user's guide for WASTOX (PB85-152882) was published in August 1984. The food chain component of WASTOX described here is a generalized model for estimating the uptake and elimination of toxic chemicals by aquatic organisms. Uptake and elimination rates are related to the bioenergetic parameters of the species encompassed in either a linear food chain or a food web. Concentrations are calculated as a function of time and age for each species included. Exposure to the toxic chemical in food is based on a consumption rate and predator-prey relationships that are specified as a function of age. Exposure to the toxic chemical in water is functionally related to the respiration rate. Steady-state concentrations may also be calculated. Food chain exposure to chemicals may be specified by the user of the model or may be taken directly from the values calculated by the exposure concentration component of WASTOX. Migratory species, as well as nonmigratory species, may be considered. The model has been successfully used to model Kepone in the James River striped bass food chain and PCBs in the Lake Michigan lake trout food chain and the Saginaw Bay, Lake Huron yellow perch food.