This study concerns the determination of phytoplanktonic pigment concentrations in the lagoon of Thau at 14 sampling stations which were surveyed 36 times during 1986. The evolution of the main pigment chlorophyll a)is described for the whole year. The object of this work is to determine if the phytoplanktonic pigment abundance is related to the nutritive richness of the lagoon water where oysters and mussels are bred.