This is a synopsis of what I've read on ferret sites:
 
What ferrets require from their food:
 
- Protein level of 32-38%  midrange being best
- fat content of 20-22%
  (as the ferret gets older, move to the lower ends of those above scales)
- Ash content no more than 7%, the lower the better
- Fiber content 3% or lower, the lower the better
 
Ingredients are listed in order of quantity, so the first three
ingredients are the most important.  The first ingredient should always be
a meat other than fish (ideally all first three ing. are meats).  Corn is
hard for ferrets to digest and should not be one of the first three ing.,
ideally it won't be in the food at all.  Check what form the meat is.
Plain meat is better than meat by product.
 
Try mixing a couple different foods to make up your ferrets daily kibble
diet.  This helps them to accept different tastes as "food" and is
important if a kibble later becomes unavailable or changes it's recipe.
 
Some of the better kibbles are
8 in 1 Ultimate, Totally Ferret, Eukanaba cat, Innova cat food.
 
Here's an 8 in 1 comparison:
Ultimate:
Protein 38%
Fat 19%
Fiber 3%
Moisture 10%
Ash 6.5%
First three ing:        Chicken, Chicken meal, ground rice
 
Ultra Blend:
Protein 35%
Fat 15%
Fiber 3%
Moisture 10%
Ash 6.5%
First three ing:         Chicken Meal, Ground Yellow Corn, Brewers Rice
[Posted in FML issue 3765]