A large, handsome slug which grows to 100-150mm long. Usually ash-black in colour it has a prominent pale keel. The mantle always lacks spots or markings but these may be present on the body and tail. The sole usually has a pale centre stripe and the mucus is clear.

Juveniles are often toffee brown with an all-white sole. The keel is always at least half the length of the body (tail to mantle) and the tentacles are generally grey and spotted.