ABSTRACT: We measured assimilation efficiencies (AEs) from ingested algal food, uptake rates from the dissolved phase, and efflux rate constants of 4 trace elements (Cd, Co, Se and Zn) in 3 size classes (1.5, 2.5, and 5.0 cm) of the mussel Mytilus edulis.AEs of Se and Zn remained constant among the 3 body sizes. AEs of Cd decreased with increasing body size, whereas AEs of Co increased with body size. The rate constant of metal uptake from the dissolved phase decreased with increasing body size, but thecalculated metal absorption efficiencies remained relatively constant or increased slightly in larger mussels. Efflux rate constants were comparable among the 3 body sizes, except for Cd which was lost about 2 times faster in the smaller mussels than inthe largest mussels. A bioenergetic-based kinetic model was used to calculate the allometric coefficient (b) of metal accumulation with mussel body size; predicted b values were comparable to field measurements. However, b increased exponentially with thegrowth rate constant, indicating that no single b value is likely to be observed in nature for a specific metal. The growth rate constant is needed for the model to predict metal concentration and allometry of metal accumulation in mussels, especially forsmaller mussels. The relative importance of trace element uptake from the dissolved phase versus particulate ingestion in the overall metal uptake in mussels was relatively independent of mussel body size, primarily because the influx rates from thedissolved phase and from food were comparably related to body size.