The brush-tailed possum, Trichosurus vulpecula, displays an unusual amount of phenotypic variation within its central nervous system. These variations are expressed in terms of gross brain morphology, as evidenced by the occurrence of both highly lissencephalic and gyrencephalic neocortices in apparently normal adult animals; by displacement and asymmetry of certain internal structures, notably within the ventral thalamic nuclei; and by the inconstant occurrence of at least one cell group within the thalamus. These variations are described and discussed with reference to possible functional, ontogenetic and phylogenetic significance.