Olfactory receptor (OR) genes are the largest gene superfamily in vertebrates. We have identified the mouse OR genes from the nearly complete Celera mouse genome by a comprehensive data mining strategy. We found 1,296 mouse OR genes (including 20% pseudogenes), which can be classified into 228 families. OR genes are distributed in 27 clusters on all mouse chromosomes except 12 and Y. One OR gene cluster matches a known locus mediating a specific anosmia, indicating the anosmia may be due directly to the loss of receptors. A large number of apparently functional 'fish-like' Class I OR genes in the mouse genome may have important roles in mammalian olfaction. Human ORs cover a similar 'receptor space' as the mouse ORs, suggesting that the human olfactory system has retained the ability to recognize a broad spectrum of chemicals even though humans have lost nearly two-thirds of the OR genes as compared to mice.