The best and safest we've found for flea control is Vet Kem.  They make a
pet spray that is kitten safe (even newborns if you spray it on a towel and
wrap it around them) and an area spray (I like the hand pump size).
 
Spray the animals starting at the neck; then the body, legs, tail; then
saturate your hands and rub it into the fur around the head, ears, chin,
etc.  Wash all bedding in hot bleach water.  Spray the cage, floor all
around the cage, furniture where the animals have access.  Let the area
spray dry before reintroducing ferrets.
 
The Vet Kem should be available from your vet, or if s/he doesn't have it
you can order it from Omaha Vaccine for about the same price.  The Vet Kem
has an ingrediant that keeps the flea eggs from hatching.  Works for up to a
month.
 
Dick B.
[Posted in FML issue 1629]