Myeloencephalopathy with diffuse Rosenthal fiber formation (Alexander's disease) was detected in a 9-month-old Scottish Terrier with progressive tetraparesis. Additional abnormalities included an electroencephalogram characteristic of an encephalopathy and a narrowed intervertebral disk space at C2-3. The diagnosis of Alexander's disease is uncommon in human and veterinary medicine. Results from this case and previous reports suggest that this disease in the dog may have a congenital or genetic basis.