The neuronal ceroid lipofuscinoses (NCLs) are inherited lysosomal storage diseases characterized by progressive neuropathy and the accumulation of autofluorescent cytoplasmic granules. Clinical signs of a new canine NCL began in a 9-month-old male Dachshund with vomiting, mental dullness, and loss of previously learned commands and rapidly progressed to include disorientation, ataxia, visual deficits, generalized myoclonic seizures, and death at 12 months of age. Neurons throughout the CNS contained autofluorescent storage granules that stained with periodic acid-Schiff and Luxol fast blue stains. Electron microscopy revealed that the storage granule contents consisted of curvilinear-appearing material characteristic of human late infantile NCL caused by CLN2 mutations. Nucleotide sequence analysis of canine TPP1, the ortholog of human CLN2, revealed a single nucleotide deletion in exon 4 which predicted a frame shift with a premature stop codon. Brain tissue from the affected dog lacked detectable activity of the tripeptidyl-peptidase enzyme encoded by TPP1, whereas the specific activities of 15 other lysosomal enzymes were higher than those in the brains of three control dogs. The affected Dachshund was homozygous for the mutant c.325delC allele, his sire and dam were heterozygotes, and 181 unrelated dogs, including 77 Dachshunds, were all homozygous for the wild-type allele. A DNA assay that detects the mutant allele will help Dachshund breeders avoid producing affected puppies in future generations. Furthermore, this Dachshund NCL may prove to be a useful model for studying the pathogenesis of neurodegeneration in human late infantile NCL and for evaluating novel therapeutic interventions for this disease.