The red flat bark beetle Cucujus cinnaberinus (Scopoli, 1763) is included as a protected species in the Berne Convention and the European Habitat Directive—Annex II and IV (92/43 EU of 21 May 1992) which requires the establishment of special areas of conservation in the European Union, and listing the species in the International Union for Conservation of Nature Red List of Threatened Species under the near-threatened category. Cucujus haematodes Erichson, 1845 is considered to be one of the most threatened saproxylic species, a relic of primeval forests, in many European countries (including Poland) under protection. Morphology of the larvae of two rare European species of the genus Cucujus (cinnaberinus and haematodes) is compared here. We point out differences which allow the two species to be properly distinguished and which, until now, have been omitted or misinterpreted in literature. The best characteristics seem to be the arrangement of minor spines on the top of basal tooth, the shape of frontal suture, size of stemmata, shape of I antennal joint, localization of basal tooth, and morphology of VIII abdominal tergites.