The sublingua of Microcebus murinus was studied gross anatomically and light microscopically. The apex of the sublingua ends in two lobe-like projections separated by a distinct median cleft. The lobes exhibit at their oral, free margin tiny processes, but no "comb"-like specializations of intensely keratinized mucosa. At the ventral surface of the sublingua three longitudinally oriented keels or ridges are found; the mucosa of the keels is strongly keratinized, so that these structures maximize the rigidity of the undertongue. In none of the specimens examined was any trace of sublingual cartilage found; in the sublingua of Cheirogaleus (only one individual examined), the nearest relative of Microcebus, cartilage was also absent. The absence of sublingual cartilage and the incidental, irregular occurrence of cartilaginous elements in the sublingua of Tarsius and Tupaia indicate that the sublingual cartilaginous skeleton is not a derivative of the hyoid arch. It is a newly developed supporting structure which does not regularly occur. A sublingua occurs only in prosimians and Tarsius; whether the fimbria linguae of the Hominoidea is the homologue of a sublingua is still disputable. The exclusive occurrence of a sublingua in prosimians and Tarsius indicates a phylogenetic relationship between these two groups and Tupaia. The "Tupaia-problem" concerns the origin of primates and from this point it derives its general importance. At present we are far from a final solution; a definite answer cannot be expected before sufficient fossil records are available which document conclusively the historical paths of the origin of primates.