Three experiments were conducted to determine the riboflavin requirement for maximal growth of young chicks. Graded levels of crystalline riboflavin were added to either riboflavin-free, purified amino acid diets or corn-soybean meal diets. Chick growth responses were obtained upon adding riboflavin to both diets. Chicks fed the purified diet required 1.8 mg of riboflavin per kg of diet, but those fed the corn-soybean meal diet required 2.63 mg of riboflavin per kg of diet. Retarded growth and leg paralysis, rather than curled-toe paralysis, were the predominant signs of riboflavin deficiency. Calculations suggested that riboflavin bioavailability in the corn-soybean meal diet was 59.1%.