Does Frontline Kill Fleas Already on Dog or Cat?

Frontline will kill all adult fleas already on a cat or dog within 12 hours of the time that they come in contact with the pesticide.

After initial application of Frontline, it takes about 24 hours for the product to spread across the entire body of the animal, at which time the protection against fleas becomes fully effective.

It will then kill all of the adult fleas on the animal, and you’ll probably see the fleas become excited and hyperactive. They usually rise to the surface of the hair instead of hiding underneath, and they become much more visible to you prior to being killed.