Three experiments assessed the effects of damage to the medial cerebellum on long-term habituation (LTH) of the acoustic startle response. Experiment 1 replicated previous results. Lesions of the cerebellar vermis blocked LTH without affecting initial response levels or short-term habituation (STH). The lesions did not disrupt LTH of a simultaneously measured lick-suppression response. In Experiment 2, vermal lesions again blocked LTH of acoustic startle. Control lesions of the cerebellar hemispheres did not affect LTH. In Experiment 3, lesions to the medial (fastigial) cerebellar nuclei blocked LTH. Lesions to the lateral (dentate and interpositus) nuclei did not affect LTH. It is concluded that the medial cerebellum (cortex and nuclei) is part of the essential circuitry for LTH of acoustic startle, whereas the lateral cerebellum is not involved in the basic habituation process.