The brain of a basking shark is described for the first time. Allometrical analysis of brain-body weight relationships indicates that Cetorhinus maximus shows by far the lowest degree of cerebralization of any shark investigated to date, possibly reflecting its plankton-feeding mode of life. Furthermore, some external morphological features of the brain appear to be species-specific. The proportions of brain parts indicate a primitive vertebrate brain organization. The size of the telencephalon, 34% of the total brain, equals that in some other sharks, whereas the cerebellum, 30% of the total brain in the basking shark, is significantly larger than in any other shark investigated. Furthermore, C. maximus shows some peculiarities concerning the nuclear expansion in the telencephalon. A rather large caudal interhemispheric region is especially striking. The relation of brain volume to brain cavity volume is approximately 1:16.