Castoreum is a yellowish exudate from the castor sacs of the mature North American beaver (Castor canadensis) and the European beaver (Castor fiber).
Beavers use castoreum in combination with urine to scent mark their territory. Both beaver sexes have a pair of castor sacs and a pair of anal glands, located in two cavities under the skin between the pelvis and the base of the tail. The castor sacs are not true glands (endocrine or exocrine) on a cellular level, hence references to these structures as preputial glands, castor glands, or scent glands are misnomers.
It is used as a tincture in some perfumes and as a food additive.
The government of Ontario pays trappers to harvest beaver castor sacs. The sacs brought CA$2.62–5.10 per ounce when auctioned at the May–June 2016 North American Fur Auction.