The ingestion of 8 algal species by oyster larvae (Crassostrea gigas Thunberg, 1793) was measured by flow cytometry (FCM). In a preliminary experiment, cell size (estimated by light scatter) and chlorophyll fluorescence of 30 algal species were evaluated to select those species which could be mixed together and still be easily discriminated by FCM. Grazing experiments were carried out over 48 h with 6 and 15-day old larvae fed on 3 algal mixtures, each containing 3 different algal species. The concentration of each species was estimated at 0, 6, 12, 24 and 48 h by FCM. Grazing pressure on a given algal species was dependent upon the age of the larvae, the time of the day and the composition of the mixture. Grazing rates of older larvae were about twice those of younger ones after 48 h (mean value of 102 and 57 cells/larvaJhour respectively). Almost no grazing activity was observed during the time interval 12- 24 h for the 6-day larvae. Significant differences between mixtures were observed after 48 hand the selective filtration of one Chaetoceros strain is of importance as this alga also proved to be of better nutritional value for oyster larvae. Data on Tetraselmis were difficult to interpret because of tigmotactic reactions of the cells.