Cage stereotypies-abnormal, repetitive, unvarying and apparently functionless behaviours-are common in many captive animals, sometimes resulting in self-injury or decreased reproductive success. However, a general mechanistic or neurophysiological understanding of cage stereotypies has proved elusive. In contrast, stereotypies in human mental disorder, or those induced by drugs or brain lesions, are well understood, and are thought to result from the disinhibition of behavioural selection by the basal ganglia. In this study, we found that the cage stereotypies of captive bank voles also correlate with signs of altered response selection by the basal ganglia. Stereotypic bar-mouthing in the caged voles correlated with inappropriate responding in extinction learning, impairments of response timing, evidence of a knowledge-action dissociation, increased rates of behavioural activation, and hyperactivity. Furthermore, all these signs intercorrelated, implicating a single underlying deficit consistent with striatal disinhibition of response selection. Bar-mouthing thus appears fundamentally similar to the stereotypies of autists, schizophrenics, and subjects treated with amphetamine or basal ganglial lesions. These results represent the first evidence for a neural substrate of cage stereotypy. They also suggest that stereotypic animals may experience novel forms of psychological distress, and that stereotypy might well represent a potential confound in many behavioural experiments.