The basal region in the developing olfactory epithelium of mice was investigated by double immunostaining using anti-keratin (MA903) and anti-bromodeoxyuridine antibodies. Basal cells proper, which are MA903 positive, and globose basal cells, which are MA903 negative, were differentiated in the basal region on embryonic day 18 to postnatal day 1. At this stage, BrdU-labeling was found more in the glubose basal cells than in the basal cells proper. The labeled globose basal cells increased in number and reached the maximum on postnatal day 3, and decreased in number on postnatal day 14. The labeled basal cells proper was concentrated near the border with the respiratory epithelium on postnatal day 1, but they were found in small numbers in other regions of olfactory epithelium during later stages of development. Active division of globose basal cells accompanied the 1.5-fold increase in the thickness of the olfactory epithelium, which is caused by the increase in the number of olfactory cells, during development. It is suggested that during late embryonic and postnatal days olfactory cells originate from globose basal cells, not from basal cells proper. The division of basal cells proper may contribute to an 8-fold increase in the surface area of the olfactory epithelium during development.