Microalgae diets supplemented with cornstarch at a rate of 0.15% of live weight to the conditioning of broodstock and spat growth of Pacific calico scallop Argopecten ventricosus were tested. To determine reproductive condition at the beginning and end of the experiment, the gonads of adults were analyzed by stereological methods. For juveniles, the weekly percent increase of spat was estimated from spat biomass volume. The microalgae diet of Tisochrysis lutea and Chaetoceros calcitrans, supplemented with cornstarch, substantially improved the reproductive status of the scallop, particularly by increasing the number of individuals in mature and mature-spawning stages with more mature oocytes by the scallop. Spat growth improved >80% in the nursery with the diet supplemented with cornstarch compared with the microalgae diet (T. lutea, C. gracilis, and C. calcitrans). It can be suggested that a small addition of cornstarch to a microalgae diets, boosts growth during nursery procedures and performance of A. ventricosus during the routine conditioning of the broodstock.