The apple maggot, Rhagoletis pomonella (Walsh), can be reared on chemically defined diets containing sucrose, amino acids (casein acid hydrolysate alone, or with 2% tryptophane, 2% arginine, 2% histidine, and 1% cystine), vitamins, and minerals. The reproductive capacity of flies fed these diets is almost double that reported previously for undefined diets.