SummaryWhen queenless colonies were each given a sealed queen cell, they built fewer additional queen cells than queenless control colonies did. However, empty queen cells or open cells with larvae did not depress the production of emergency cells. The experiments indicate that an immature queen in a sealed cell produced a pheromone with inhibitory activity on the initiation of queen rearing. This activity was less than that of an adult queen, but greater than that of synthetic9-oxo-trans-2-decenoic acid.