During the first week of adult life the olfactory system of the honey bee undergoes a critical period of maturation [Masson and Arnold, Organisation and plasticity of the olfactory system of the honeybee, Apis mellifera, in: Menzel and Mercer (Eds.), Neurobiology and Behaviour of Honeybees. Springer-Verlag, Berlin, 1987, pp. 280 295]. This is accompanied by dramatic increases in the volume of the antennal lobes [Winnington et al., Structural plasticity of identified glomeruli in the antennal lobes of the adult worker honey bee. J. Comp. Neurol., 365 (1996) 479-490], centres of the brain that receive direct input from primary olfactory receptor neurons housed in the antennae of the bee. Here, we show that during the first 4-6 days of adult life there is a significant increase in the percentage of bees that respond to a conditioned olfactory stimulus after a single conditioning trial and, furthermore, that the ontogeny of this olfactory learning behaviour is altered significantly if the queen is removed from the colony. The absence of a queen during early adult life also has site-specific effects on the maturation of the antennal lobes of the brain. These results show for the first time that the queen's presence in a colony has a significant impact not only on the behaviour of the adult worker honey bee, but also on the structure of the brain.