Oyster farming in France is a traditional activity. Each year, 60,000 tons of C. gigas are fattened before being sold. Present-day fattening techniques for improving oyster taste and colour and increasing the meat weight are not particularly reliable. To optimize the fattening process, large phytoplanktonic blooms were induced in ponds, then distributed in oyster ponds. Despite the variability of the autumn weather conditions, diatoms (Skeletonema costatum) have been successfully cultured in outdoor ponds. During neap tides (when no seawater replenishes the water in the oyster beds), between mid-October and mid-December 1996, the fattening index (the weight of the meat) was constant or increased by 10-15% when a supplement of algae of 380.000 cells/oyster/day was added to the ponds. Algal supplement was clearly responsible for fattening improvement. In 1995, the addition of 110,000 cells/oyster/day, a significantly lower quantity, was not sufficient to prevent weight loss. Over two periods of 3 months (in 1995 and 1996) of the experiment, a significant reduction in weight (up to 20%) was observed in oysters grown in ponds which did not receive additional algae.