When we moved into our house, we had one flea-less indoor cat. However, the previous owners had originally had 3 indoor/outdoor cats, but only one cat when they moved out (one was hit by a car and they gave the other one to their son). Long story short, we had a houseful of fleas all converging on our one poor kitty.:-( We used Advantage on her and ended up having the house professionally flea-bombed (had to leave the house for the afternoon, etc). Granted, it was a drastic measure, but I was even getting bitten by the little buggers all the time unless I used bug spray on myself - YUCK! After that, we haven't seen a flea since!