Honey bee larval growth is markedly influenced by the moisture content of the larval food. A gradual increase in the moisture content of the larval food (royal jelly) resulted in a large percentage of queens. An apparent linear relationship of royal jelly moisture content to time of larval development was used in the feeding trials. Since the larval food tested was one and one-half or two and one-half years old, it is probable that a highly labile queen determining substance, or substances, if present in larval food, is not responsible for caste determination. Results of this study suggest that nurse bees may initiate the mechanism of queen differentiation simply by increasing the moisture content of the food of growing larvae and, thereby, controlling the intake of essential nutrients. The secretions of the mandibular glands are presumably responsible for the change in consistency of the food of either queen or worker larvae.