Vertebrate odorant receptor (OR) genes are divided phylogenetically into two distinct classes: the fish-like class I and the terrestrial-specific class II. In the present study, we systematically analysed mouse class I OR genes (42 subfamilies) to elucidate the expression profiles in the olfactory epithelium (OE) and the projection sites of their olfactory sensory neurons (OSNs) in the olfactory bulb (OB). In situ hybridization (ISH) revealed that most class I OR genes (36 subfamilies) were expressed in the dorso-medial zone (zone 1) of the OE. Furthermore, there appeared to be no significant differences in the distributions of OSNs expressing class I genes within zone 1. These results indicate that there is a clear boundary between zone 1 and non-zone 1 areas in the OE. Some class I ORs are known to possess ligand specificity for aliphatic acids, aldehydes and alcohols. Our ISH analysis has revealed that OSNs expressing the class I ORs in zone 1 tend to converge their axons on a cluster of glomeruli in an antero-dorsal domain that is assumed to be involved in responses to the aliphatic compounds on the OB.