When I found out that Belle had smelly ears we went right to the vet. They said that was the first sign of infection and you have to have antibiotics to get rid of it. If they don't smell and they scratch alot or shake their heads, it could be ear mites. You can get stuff from the vet or at a pet shop. I used the stuff from the pet store on my Schnauzer and it worked fine. don't wait, infections can get really bad.
