I would be careful about Flea products.....use Frontline or Advantix brands - NO HARTZ ...they are very toxic to small animals!! Also - DAWN Dish Washing Detergent will kill fleas on contact....you can add it to your yorkies normal shampoo and it's susposed to kill them immediately...Dawn is becoming very popular for Flea control -
Here is a link -
http://my.homewithgod.com/mkcathy/household/animal.html
Also - I copied this from that site -
Kills fleas instantly. Dawn Dish Washing Liquid does the trick. Add a few drops to your dog's bath and shampoo the animal thoroughly. Rinse well to avoid skin irritations. Good-bye fleas. You can also put a bowl of water with dawn in it under a light which will attract them and they drown.