The present investigation was undertaken to fill in the gaps in the embryology of Tenebrio molitor, shown by previous work to be a relatively unspecialized beetle. The early development, which is typically coleopteran, is briefly described. Middle and lateral plates are delimited along the length of the germ band and the middle plate invaginates to give rise to the mesodermal inner layer. Invagination commences and is most pronounced posteriorly, and progresses anteriorly. Atypically for endopterygotes, the pre-oral mesoderm arises in situ. Commencing in the anterior region of the protocorm the mesoderm spreads out laterally, between the ectoderm and the yolk. It segments prior to the ectoderm, from the labral to the tenth abdominal segment. Somites are formed laterally, in the typical manner. In the eleventh abdominal segment the undivided mass of mesoderm gives rise to the proctodaeal splanchnic musculature. All the somites become excavated into coelomic sacs, of which there are three protocephalic, three gnathal, three thoracic and ten abdominal pairs. Labral coelomic sacs are described for the first time in the Coleoptera; this is the second recorded instance of their occurrence in the Endopterygota. The intercalary coelomic sacs, which are generally suppressed in the endopterygotes, are median and intersegmental in position and give rise to the suboesophageal body. The structure of each cephalic coelomic sac is described. The labial and trunk sacs are similar, and resemble those in other Coleoptera. Mesodermal segmentation, especially in the cephalic region, is better developed in T. molitor than in the Coleoptera so far described. It is thus more suitable for head segmentation studies than the beetles hitherto subjected to such an investigation. The in situ origin of the pre-oral mesoderm, the complete intersegmental separation of all but the terminal somites and the occurrence of well developed coelomic sacs in the protocephalon are to be regarded as primitive features in a holometabolan, indicating exopterygote affinities.