We evaluated the effect of nine isocaloric diets prepared with a mixture of microalgae alone (Tisochrysis lutea + Chaetoceros calcitrans + Ch. gracilis; 1:1:1) and different combinations of the algal mixture and cornstarch or wheat flour, on the Scope for Growth (SFG) of Cortez oyster Crassostrea corteziensis juveniles (7.2 ± 1.1 mm shell length). A total 15,000 spat was acclimated to the experimental diets for 30 days at 21°C, prior SFG determinations. Consumed energy, absorbed energy, and SFG decreased with a corresponding decrease in the proportion of live microalgae in the diet. Oysters fed the microalgal diet showed significantly increased physiological activity in all parameters except absorption efficiency. Oysters fed diets containing up to 50% dry feedstuff showed positive SFG values but these were significantly lower than SFG shown in oysters fed microalgae alone. Oysters fed diets containing ≥75% dry feedstuff showed negative SFG. These results suggest that cereals can be used as complementary food or C. corteziensis spat in hatchery operations, provided replacement of microalgae is kept below 50%.