A 9-week-old Jack Russell terrier with progressive ataxia had histopathological lesions consistent with neuroaxonal dystrophy. Gross observation revealed absence of the septum pellucidum, hypoplasia of the corpus callosum and marked bilateral hydrocephalus. Light microscopy of the CNS revealed extensive axonal swellings principally in the gray matter of the brain stem where the sensory nuclei were most affected. Ultrastructurally, spheroids were identified as axonal terminals and probably dystrophic boutons; they were characterized by accumulations of membrane bound bodies. Clinical and morphological findings are similar to those identified in human infantile neuroaxonal dystrophy (Seitelberger's disease).