As members of the weasel family, ferrets have their own distinctive smell. "Descenting" (removing the anal glands) and spaying or neutering your ferret will decrease the smell, but scent glands in their skin still produce a natural oily substance that adds to your ferret's unique scent. Bathing your ferret frequently with a mild shampoo designed for ferrets may temporarily decrease the odor but can also cause the skin to dry out or the glands to produce even more oil in response.
If the smell seems to come from your ferret's ears, you may want to check for ear mites as ferrets do produce large amounts of ear wax and may be prone to ear mites.
Most of your pet ferret's odor probably comes from his cage and litter box. Keeping these clean will go a long way to controlling your ferret's odor and increasing your enjoyment of your pet.
