The adult fleas that you see on your pet only represent 5% of the total flea population. For every adult flea you see, there are about 20 more fleas in the immature stages (eggs, larvae, pupae) living in your pet's environment. It is important that flea products for both the pet and the pet's environment include an adulticide that kills the adult flea, and an insect growth regulator that kills the immature stages. All household pets should be treated at the same time to ensure optimal results.