We have studied the behavior of a new neurological mouse mutant, the hot-foot mutant. The most obvious phenotypic characteristic of the mutation is an abnormality of posture (body flattened on the ground, back limbs spread wide) and of locomotion (backing up and jerky movements of the hind limbs). Neuroanatomical analysis of the cerebellum showed abnormalities mainly related to the Purkinje cells, which have ectopic spines devoid of presynaptic innervations. The correspondence between the cerebellar alterations and the abnormalities of locomotion is thus clearly illustrated. However, the mild degree of the anatomical alterations does not seem sufficient to account for the degree of effect on locomotion and posture.