Replacement of adult maxillary jaws is reported for Diopatra aciculata. The process is similar to arthropod moulting, commencing with apolysis or retraction of the epidermal cells from the inner surface of the old cuticle, followed by formation of the new cuticle. This results in the jaw-in-jaw or pharate state. Ecdysis or shedding of the old cuticle takes place without any splitting or damage as evidenced by complete, fully articulated shed jaws found in the gut. Newly moulted maxillae are soft and white until they become sclerotized. The new jaws are about 1.4 times the size of the old ones (moult increment). The same process is expected to occur in all extinct and extant Labidognatha. The Recent Lumbrineridae maxillae may not be of the labidognath type, but probably moult in the same manner. Occasional finds of pharate jaws of the extinct Placognatha indicate that they underwent similar jaw replacement. Not enough is known about Prionognatha, although shedding before replacement may occur. Jaw replacement in Ctenognatha, where the new elements form in sac-like epithelial structures ventrolateral to the existing maxillae, is probably limited to this group.