The olfactory system is unique in that the sensory input is in the form of molecular information carried in a vast variety of odorants. Nearly 1000 types of odorant receptors mediate the initial detection and discrimination of odorants at the molecular-feature level. The discrimination at the molecular level is converted into that at the cellular level (olfactory sensory neurons) by the one sensory neuron-one odorant receptor rule, and then into that at the neuronal circuit level in the olfactory bulb by the specific olfactory axon connectivity pattern. Individual glomeruli in the olfactory bulb represent a single odorant receptor, and the glomerular sheet at the olfactory bulb surface forms odorant receptor maps. This review focuses on the spatial organization of the glomerular sensory map in the olfactory bulb. The analysis using the optical imaging method suggests that odorant receptors having a common molecular-feature receptive site are grouped together and represented by glomeruli that are localized in topographically fixed domains in the olfactory bulb. The domain organization may be a structural unit for the spatial organization of the glomerular sensory map, and might relate to the olfactory submodality.