Bivalve hemocyte responses are modulated by numerous biotic and abiotic environmental factors. This study investigates how nutritional stress canmodulate two hemocyte functions classically studied in bivalves, i.e., phagocytic and oxidative activities. For this purpose, we exposed zebra mussels (Dreissena polymorpha) to three nutritional conditions, for 42 days: (1) one groupwas fedwith 1×10⁶ algal cells permussel and per day, (2) a second groupwas fed with 10%of the previous food intake (1×10⁵ cells per mussel and per day), and (3) the third one was deprived of food. Hemocyte responses were assessed by flow cytometry every week for 42 days. Food deprivation was associated with a significant decrease in granulocyte size. Phagocytic activity increased in mussels exposed to the three diets, but it was more pronounced in mussels partially deprived of food (10% food intake). Mussels exposed to stressful nutritional conditions (10% and 0%) displayed significantly decreased oxidative activity from 14 days of exposure, whereas those fed on a normal diet displayed stable intracellular oxidative activity throughout the experiment. This study shows that nutritional conditions affect hemocyte morphometry and activity. Zebra mussel sensitivity to the nutritional conditions seemed low, even with total food deprivation for 42 days. It is necessary to estimate the physiological consequences of these food conditions, especially on the energetic status of this bivalve organism but also on its ability to resist to infection.