Callista chione is the most prominent suspension-feeding bivalve species inhabiting the soft bottoms of the Bay of Banyuls-sur-Mer. Ingestion rates and absorption efficiencies when fed monospecific cultures of either phytoplankton (Pavlova lutheri) or bacteria (one natural strain isolated from the marine environment) were determined using 14C radioactive pulse–chase feeding experiments. Microalgae were ingested at a rate five to six times higher than bacteria (i.e. 2.31×10−3 against 0.40×10−3 mg DW mg SFDW−1 h−1). Such differences were attributed to differences in average particle sizes. Bacteria were also less absorbed than microalgae (i.e. 64.3 vs 83.8%). Together with existing data regarding the ratio of free to attached bacteria in the oceanographic environment, these results suggest that free bacteria probably contribute to meet only a small portion of the energy requirements of adult C. chione populations.