A mass-balance model for the uptake of organic contaminants in the opposum shrimp (Mysis relicta) is developed. The model describes the concentration in the mysid as a function of time as it grows and its lipid content changes over a two-year life span. The model can describe uptake in varying proportions from pelagic and benthic sources. Four mysid scenarios are considered: entirely pelagic, entirely benthic, half-pelagic and half-benthic, and following the observed seasonal behavior patterns. The seasonal-scenario mysid yields results consistent with levels of total polychlorinated biphenyls reported for Lake Ontario (Canada/USA). The relative sediment and water fugacities are shown to play a critical role in determining the contamination level in mysids. Inclusion of mysids in a simple food-web model demonstrates higher concentrations in upper-trophic-level organisms by two effects: introduction of another trophic level in the food web, and increased benthic-pelagic coupling.