During a 4-week experiment on oyster grazing in coastal ponds (claires) near La Rochelle (France), a red coloration was observed in the digestive gland of oysters. This pigmentation was attributed to an intensive grazing on cryptophyte nanoflagellates. Three different techniques led to this conclusion, mentioning for the first time free-living cryptophytes as responsible for such a phenomenon. Spectrofluorometry was used to demonstrate that the red coloration was due to the presence of a phycoerythrin, characteristic of cryptophytes. Microscopic observations and accessory pigments analyses in water ponds confirmed that the source of this pigment was of cryptomonadal origin. Oyster grazing on these algae is evidenced by large differences in alloxanthin concentrations and flagellate abundances between the two experimental ponds. The winter occurrence of this phenomenon is of importance for oyster commercialization and a short-term depuration is suggested to remove the undesirable pigmentation. D