Yep - definitely agree with Sam, and worth reiterating for everyone - never use frontline on rabbits, it can kill them! I've had rabbits with cat fleas in the past, it's fairly obvious to tell, in much the same way as it is for a cat. As with cats, it's really important to treat early even if they are outside, as being smaller (usually!) they can get seriously anaemic very quickly. Rabbits can also get rabbit fleas, but these tend to accumulate in groups at the base of the ear, rather than running around like cat fleas do.