Go to the vet and get Advantage for cats. It lasts about 1 month, but needs to used 3 months in a row, applied on the same day each month. It worked wonders for all 5 of my cats and dog. Most where gone in about 1 day.

 If you have kittens full of fleas, take baby oil and wash them in it, flea's will kill kittens if theres a lot on them,but for a cat yes advantage