This study has been carried out on two adults of the species Sphenodon punctatus (Sphenodontidae). The results allowed us to establish the numerical features of the brain organization in this species. Comparisons were done with regard to a sample of forty various species of lizards, in which six species of Lacertids were taken as a mean reference level (Platel, 1975). Opposite to those of the Lacertids (100), the index of encephalization in Tuatara has a low value (70.2), but locates Sphenodon in the neighbourhood of lizards of the family Cordylidae (Cordylus, Zonosaurus). This feature remains the same for the main subdivisions of the brain, according to the value of their structure index (as well as their isoponderal percentage). In most subdivisions, the indices show also low values, except for Epithalamus (359), dorsal thalamus (118) and cerebellum (121); some hypothesis were given according to olfaction, vision and locomotion in Tuatara. On the whole, our results lead us to consider the Tuatara as a few evoluted Lepidosaur (according to its brain organization); comparisons with lizards bring it close to the Lacertomorpha (Type I Lizards), considered by Northcutt (1972) as less advanced saurens than the Dracomorpha (Type II Lizards).