The morphology of the claustrum was studied by stereological methods in representatives of five mammalian orders (Insectivora, Rodentia, Lagomorpha, Carnivora and Primates). In each species under study, a dorsal and a ventral part of the nucleus can be distinguished. Based on differences in shape and separation from surrounding structures, five morphological types of the claustrum occur. The claustrum of Insectivora and some rodents represents the least complicated morphological type. The nucleus is very poorly separated from the surrounding structures. The human claustrum is morphologically the most complicated, although the two above-mentioned principal divisions are apparent. The ventrally situated paraamygdalar part of the human claustrum may correspond to the endopiriform nucleus or ventral part of the claustrum of other mammals, because of its morphological characteristics and connections with the limbic system. In guinea pigs, traditionally classified as members of the Rodentia, a characteristic morphological type of the claustrum is present. This observation may support arguments questioning the current position of this species in mammalian classification. Based on stereological studies, the increase of the claustral volume that occurs with increase of the hemispheric volume is significantly smaller than the increase of the isocortical volume and larger than the increase of the allocortical volume. The increase of the volume of the dorsal and ventral parts of the claustrum does not differ significantly in the species under study. Neurons of the claustrum represent differentiated morphology. The numerical density of neurons in the dorsal part of the claustrum is significantly higher than in the ventral one. Differences in the morphology and cellular structure of the two parts of the claustrum may suggest differences in function of the two parts of the nucleus, most probably concerned with transfer of information among various cortical regions. Changes in the claustrum, a cortico-related structure, that occur with increased brain volume, may suggest that its development is less dynamic than that of the isocortex.