Revolution only kills adult fleas and prevents eggs from hatching, according to the enclosed pamphlet. So you have to wait until any larvae in the environment develop into adult fleas and bite your pet before those fleas die. I wish I knew this before I used Revolution on all my three cats. First I used Advantage and that simply does not work any more. Seems fleas are immune now. So vet said it was ok to use Revolution a few days later. But one week later I still have fleas on my cats and don't feel it is safe to use another product until a month has passed. This is a drag.