Relative retention efficiency (Er%) of Crassostrea gigas was estimated in the laboratory with 7 diets consisting of algae and silt. Total particulates varied from 1.34 mg/l in natural sea water to 27.65 mg/l in natural sea water enriched with silt. It was also calculated from field experiment to get high concentration of seston up to 66 mg/l. It appeared that Crassostrea gigas did not adjust its retention according to the food quality and the gill porosity was controlled by the concentration of particulate matter. In low seston load (1.34 mg/l) the oyster showed no change in retention for particles larger than 3-4 µm (Equivalent Spherical Diameter, ESD) but at the highest concentration (66 mg/l) it could only retain with a maximum efficiency particles larger than 12 µm (ESD). The japanese oyster having a passive response to high concentration, a model where suspended particulate matter was assessed by Particulate Volume, PV, (mm3/1), was deterrnined: Er%=(-12.537e(-0.304*ESD))PV + 100