Within the framework of an investigation into the carrying capacity of the Bay of Marennes-Oléron (France) for bivalve culture, the in situ uptake of suspended particulate material by oysters (Crassostrea gigas) and mussels (Mytilus edulis) was determined in experiments with benthic ecosystem tunnels. Very high fluctuations in seston quantity and quality were observed within and between tidal cycles. The percentage of organic carbon was inversely related to seston quantity at low concentrations. Organic carbon was diluted by resuspension of material rich in inorganic matter. At high seston concentrations a constant level of about 2% organic carbon was found. The CIN ratio was relatively constant throughout the seasons and fairly low (6.5 to 8.4). Owing to the presence of bivalves large fluxes of suspended particulate material were observed in the in situ measurements. Selective retention of organic carbon or nitrogen could not be demonstrated. Clearance rates based on chlorophyll uptake were within a normal range compared to other studies. A large contribution to the food of the bivalves seemed to be formed by resuspended microphytobenthos. Judged by the low CIN ratio, the food was of good quality. Although its quantity was. variable by dilution, it may support largely the carrying capacity of the Bay of Marennes-Oléron for the cultivation of bivalves.