Using Immunohistochemical techniques, olfactory marker protein (OMP) was localized to the main (MOB) and accessory (AOB) olfactory bulbs of 30− and 45-day-old, and adult Brazilian opossums, Monodelphis domestica. Entire olfactory nerve and glomerular layers of the adult opossum MOB were darkly stained. In the adult AOB the rostral half of these two layers was stained more intensely than the caudal half, and both parts were less darkly stained than the MOB. This differential AOB staining was not present at 30 days of age, but was evident by postnatal day 45, although not as dramatic as in adults. This is the first report of differential OMP expression and may provide an approach to identifying the function of OMP.