CDC stands for Cul Du Canard which is the back end of a duck. These are small puffy downy feathers that surround the oil glands. The duck dips its bill into these oily feathers and then spreads a thin layer of oil on all of its other feathers to waterproof them. What does this have to do with fly fishing? These special oily feathers are used in a variety of patterns because they repel water. When used on dry flies, they can be used to make a bouyant wing, and when used on wet flies the feather stays dry underwater and has a very lively action. The CDC feathers also trap air bubbles in their fibers, which simulates the air bubbles that naturally come off of nymphs as they are emerging. 

The CDC France Fly is a Euro-style nymph tied with an oversized tungsten bead head. This little guy will get down deep quickly and will hook many a trout.