The skeletal anatomy of the beetle Tenebrio molitor L. was investigated. The head is of the generalized tenebrionid type, with prognathous mouthparts concealed dorsally by an enlarged epistoma. The frontoclypeal region is ill defined because of the shifting of the origins of some of the stomodeal muscles. Similar changes have been described in other beetles, including several tenebrionids. It is believed, however, that the epistomal suture in this family is homologous to the frontoclypeal and genoclypeal sutures of other insects. The most complex area of the cranium is the gular region, which appears to have been formed by the ventral extension of the postoccipital sutures and more anteriorly by a medial encroachment of the postgenae. The structure of the prothorax is complicated by the invagination of the ventral surface to enclose the coxae and the ventral extension of the tergum, eliminating the pleura except for the trochantins and pleural apophyses. The mesothorax is distinguished, as in other Coleoptera, by the reduction of sclerites in the axillary region and by the deep invagination of the membrane behind the epimeron to form a pocket for the metathoracic spiracle. The metathorax is of the typical coleopterous type but is partially desclerotized, along with the anterior abdominal terga, indicating the possible onset of flightlessness in Tenebrio. A series of intricate locking mechanisms serving to hold the elytra in place when the beetle is at rest are described. These involve the mesothorax, metathorax, and abdomen. The abdomen is largely membranous dorsally. Sternites 2 through 5 are fused and immovable as in other tenebrionids. A pair of small, weakly sclerotized plates in the metacoxal region is shown to represent the first abdominal sternite, previously believed to be absent in the higher Coleoptera. The genitalia are similar to those of Tribolium, with an elaborate system of telescoping membranes and sclerites in the male. Well-developed paraprocts are present in some females, perhaps representing the styli of the coxites of the ninth abdominal segment. A previously undescribed secondary sexual character, the development of the medial protibial apex as a spur in the male, is described.