Mussels have been widely used as bioindicators of coastal contamination. Laboratory experiments employing radiotracer methodology were conducted to determine the assimilation efficiencies (AE) from ingested phytoplankton, the influx rates from dissolved source and the efflux rates of seven trace elements (Ag, Am, Cd, Co, Cr, Se and Zn) in the mussel, Mytilus edulis. A bioenergetic-based kinetic model was then employed to predict trace element concentration in mussel tissues in two locations for which mussel and environmental data are well described: South San Francisco Bay (CA) and Long Island Sound (NY). Elemental cellular distribution in phytoplankton, elemental gut passage time in mussels and metal desorption under acidic gut environments of mussels are all major variables affecting trace element assimilation in mussels. AEs of trace elements decreased inversely with mussel ingestion rate. In experiments with 7 species of phytoplankton as diets for mussels, AEs of Cd, Se and Zn were directly correlated with organic carbon assimilation. AEs of Am, Co and Se also increased with elemental penetration into the algal cytoplasm. Assimilation of all trace elements (except Cr) and carbon increased with their gut passage time. Cd assimilation increased with increasing Cd concentration in diatom cells, whereas Zn assimilation was inversely related to diatom Zn concentration. The influx rate of elements from the dissolved phase increased with the dissolved concentration. The calculated dissolved uptake rate constant was greatest for Ag, followed by Zn >Am Cd > Co > Cr(VI) > Se = Cr(III). Salinity had an inverse effect on the influx rate from the dissolved phase and dissolved organic carbon concentration had no significant effect on trace element uptake. The calculated efflux rate constants for all elements ranged from 0.01 to 0.03 day-1. The route of trace element uptake (food vs. dissolved) and the duration of exposure to dissolved trace elements (12 h vs. 6 d) did not significantly influence trace element efflux rates. A model which used the experimentally determined influx and efflux rates predicted concentrations of Ag, Cd, Cr, Se and Zn in mussels that were directly comparable to actual tissue concentrations measured independently in two reference sites. Sensitivity analysis indicated that the total suspended solids load can significantly influence metal bioaccumulation for particle-reactive elements (Ag and Am). The model predicted that under conditions typical of coastal waters over 96% of Se in mussels is obtained from ingested food. For Ag, Am, Cd, Co and Zn, the relative contribution from the dissolved phase decreases significantly with increasing metal partition coefficients and AEs; values range between 33 and 67% for Ag, 5 and 17% for Am, 47 and 82% for Cd, 4 and 30% for Co, and 17 and 51% for Zn. The majority of Cr (62 - 87%) in mussels arises from food ingestion of Cr(III), whereas only 13 - 38% is from uptake of Cr(VI) from the dissolved phase. In summary, the bioenergetic kinetic model developed here can be applied to elucidate the rates and routes of metal accumulation in mussels. Such information can be used to interpret the voluminous data being generated by national monitoring programs, and to set a new generation of water quality criteria.