The contents of glucose, fructose and sucrose were determined in the natural diets of male and female honeybee (Apis mellifera) larvae reared under various conditions. Glucose was predominant during the early larval stages of workers and drones, but fructose became the main sugar component in the food of older larvae. In the diet of queen larvae, glucose remained the main sugar component throughout the larval period. The abundance of food supplied to worker larvae after a broodless period was characterized by an extremely low glucose content. The significance for caste development of these marked proportional differences in glucose and fructose in larval diets is discussed.