Pulse-chase feeding and multi-labeled radiotracer techniques were employed to measure the assimilation of 6 trace elements (^110mAg, ²⁴¹Am, ¹⁰⁹Cd, ⁵⁷Co, ⁷⁵Se and ⁶⁵Zn) from ingested diatoms in the mussel Mytilus edulis feeding at different rates (0.1, 0.49 and 1.5 mg dry wt h^-1). Uniformly radiolabeled diatoms Thalassiosira pseudonana were fed to mussels for 0.5 h, and the behavior of the radiotracers in individual mussels was followed for 96 h in a depuration seawater system. Assimilation efficiency (AE) of each element declined with increasing ingestion rate and increased with gut passage time. The importance of extracellular digestion relative to intracellular digestion increased with ingestion activity, which, when coupled with a decline in AE, suggested that extracellular digestion is less efficient in metal absorption. Zn assimilation was most affected by ingestion rate, suggesting that AE may play a role in the physiological regulation of this metal in M. edulis. In an experiment to simulate the effects of an acidic gut, lowered pH (5.5) enhanced the release of elements from intact diatom cells, especially at low particle concentration. These results indicate that both feeding components of the mussel (i.e. gut passage time, digestive partitioning) and metal chemistry (i.e. metal release at lowered pH within the bivalve gut) are responsible for the difference in the assimilation of trace metals at different food quantities observed in mussels.