Just forget the stuff you find in stores, specially Hartz products, there has been some problems with animals getting sick/dieing in the past... buy either from a vet or online, there are several places you can buy it online for cheaper then you might find at most vets.
Are the only ones I would use on my fur kids, and they have worked WONDERFULLY.
All 3 of my cats are indoors, but sometimes foster animals bring fleas or ticks in, etc.
Just do some searching on the web, and you'll find which one is best for you, some of them prevent other things besides just flease, they are different. Good luck!