Zebrafish, Danio rerio, possess a well-developed sense of smell which governs a variety of behaviors. Both the number of odorant receptor genes and the number of modules in the olfactory bulb (glomeruli) are about an order of magnitude smaller than those of mammals. Nevertheless, the spatial organization of functional properties within the sensory surface and the olfactory bulb are comparable to those of mammals. The quantitatively reduced olfactory system of zebrafish, together with the suitability of this species for developmental and genetic studies, make zebrafish an interesting model system to study olfactory differentiation and neuronal representation of olfactory information.