Some of the particulate matter found in bottom sediment comes from material rejected as pseudofaeces or produces as faeces by Crassostrea gigas . The amount of pseudofaeces rejected is proportional to the amount of seston for total seston values up to 4 .6 mg 1 -' . For this values up to pseudofaeces production, the amount of faeces is mainly limitated by pseudofaeces production and reaches a constant mean value of 8 .9 mg 1 -' g DW -' when seston concentration increases .The proportion of faeces versus pseudofaeces is over 1 when seston concentrations are below 10 mg 1 -', and remains less than 1 when seston concentrations are higher than 10 mg l -' .The pseudofaeces show a lower organic content (13 ± 3 %) than the material trapped through the gills (27 ± 4%), as well as a lower percentage of proteins and lipids, suggesting a selection by the labial palps . On the contrary, an enrichment with labile carbohydrates is recorded in pseudofaeces, probably due to mucus agglutination of rejected particulate matter .The feeding of Crassostrea gigas can package and transform (agglutinate, change the organic content, change the biochemical composition) suspended matter, thus influencing both the quality and quantity of organic matter sedimentation .