Oyster farming in France is a traditional activity. Each year, 149 000 tons of oysters are fattened before being sold. More and more oyster farmers supplement the diet of oysters by microalgae to optimize the fattening process and to improve both the growth and flesh quality of oysters. In the present study, oysters were supplemented by two microalgae: Skeletonema costatum and Tahitian isochrysis clone. The ash, protein, carbohydrate (including glycogen), and lipid contents were analyzed. The growth of oysters was improved when they were fattened with microalgae, leading to an increase in the condition index. The chemical composition of oysters was influenced by the chemical composition of the microalgae, especially the level of glycogen, which was significantly increased for oysters supplemented by microalgae. The fatty acid profile of oysters fattened by microalgae was positively correlated with the fatty acid profiles of S. costatum and T. isochrysis. These results show the effectiveness of supplementation by microalgae on the growth and on the biochemical composition (glycogen and fatty acids especially) of oysters.