Please do NOT use Revolution on your yorkie. Many yorkies have had bad reactions to this product. The vets love it because it covers EVERYTHING from fleas, to ticks, to worms and heartworm. It is a CHEMICAL and I totally do NOT recommend it. If you decide to use this product on your yorkie, I cannot stand behind your yorkie with my guarantee. I recommend either Interceptor or Heartguard for heartworms and regular worms for your yorkie.