Although it has been shown that trophic transfer of trace elements in oysters can be influenced by the diet, most of the studies investigating the ability of oysters to bioaccumulate trace elements from their diet are based on experiments using phytoplankton alone. Wild oysters feed also on large bacteria, ciliates or detritic organic matter. The present study aimed at examining the influence of quality of food on the assimilation efficiency (AE) of trace elements in the Pacific cupped oyster Crassostrea gigas. Oysters were exposed via their food to the radiotracers of essential (57Co, 54Mn and 65Zn) and non-essential (110mAg, 109Cd and 241Am) trace elements under different diets (protozoan ciliates Uronema marinum and diatoms Thalassiosira pseudonana). Significant differences were found only for Ag and 241Am, with lower AEs measured in oysters fed with ciliates than in individuals fed with diatoms (Ag: 54 ± 3% vs. 67 ± 4% for Ag and 241Am: 62 ± 4% vs. 76 ± 4%). Interestingly, no significant difference was found among estimated depuration rates (kel) for all trace elements ingested with the two diets tested. These findings indicate that the differences observed are driven by the digestion process, presumably due to difference of bioavailability of trace elements dependent on the quality of the food ingested.