Most animals depend on the identification of odours to locate food or to find mating partners. To accomplish this, the olfactory system must recognize relative concentrations of a large number of substances by analysing complex patterns of chemoreceptor activations(1,2), but how these patterns are represented in the brain is not well understood. Previous studies indicated that odours evoke specific patterns of activity in olfactory sensory centres(3-7) and led to the hypothesis that single glomeruli in the olfactory bulb of mammals respond to particular receptor types(8-10). We made optical recordings in vivo in the honeybee brain to investigate neuronal population responses to odorants delivered naturally to the animal. We report here that odours evoked specific spatio-temporal excitation patterns in the antennal lobe, the structural and functional analogue of the olfactory bulb(11). Specific ensembles of active glomeruli represent odours in a combinatorial manner. A comparison between different individuals shows remarkable similarities for a pheromone component, but not for general flower odours. Mixtures evoked patterns that were combinations of the single odorant responses. These combinations were not fully additive, however, indicating inhibitory effects on single glomeruli. Such interactions could be crucial for the formation of singular codes for complex odour blends.