To determine the effects of food quality on Ostrea edulis reproduction, European flat oysters were conditioned during two sets of experiments, carried out in spring and autumn, during 40 days at 19 degrees C, in 50 l transparent flow-through tanks, in triplicate, and fed constantly at 900 mu m(3) mu l(-1), with eight different types of microalgae. Four species were fed per group of trials: Isochrysis affinis galbana, Chaetoceros gracilis, Skeletonema marinoi, and Tetraselmis suecica were fed to flat oysters in the first set; whereas Rhodomonas salina, Thalassiosira weissflogii, Thalassiosira pseudonana, and Pavlova lutheri were provided during the second set of experiments. At the beginning and end of both conditioning periods, oysters were sampled and processed for histology analysis for each diet. Each oyster was classified for its sex and gonad development stage. Oysters fed S. marinoi and C. gracilis exhibited the highest ratio of hermaphrodites with 96 and 77% respectively, whereas those fed T. suecica showed the lowest hermaphrodite percentage, 59%. When oysters were conditioned throughout the second set of experiments with four other species, oysters fed R. salina and T. weissflogii exhibited 83 and 87% of hermaphrodites. Regardless of the diet, a gonad development occurred during the first set of experiments with >= 60% ripe oysters (stage 3) and spawned oysters (stage 4). In the second set of trials, oysters fed R. salina and T. weissflogii were highly mature with 90% and 75% of stage 3 and stage 4 respectively; whereas those fed P. lutheri showed low maturation with only 17% of ripe oysters.