Mutations of the neuronal sodium channel gene SCN8A are associated with lethal movement disorders in the mouse and with human epileptic encephalopathy. We describe a spontaneous mouse mutation, Scn8a(9J), that is associated with a chronic movement disorder with early onset tremor and adult onset dystonia. Scn8a(9J) homozygotes have a shortened lifespan, with only 50% of mutants surviving beyond 6months of age. The 3bp in-frame deletion removes 1 of the 3 adjacent isoleucine residues in transmembrane segment DIVS6 of Nav1.6 (p.Ile1750del). The altered helical orientation of the transmembrane segment displaces pore-lining amino acids with important roles in channel activation and inactivation. The predicted impact on channel activity was confirmed by analysis of cerebellar Purkinje neurons from mutant mice, which lack spontaneous and induced repetitive firing. In a heterologous expression system, the activity of the mutant channel was below the threshold for detection. Observations of decreased nerve conduction velocity and impaired behavior in an open field are also consistent with reduced activity of Nav1.6. The Nav1.6Δ1750 protein is only partially glycosylated. The abundance of mutant Nav1.6 is reduced at nodes of Ranvier and is not detectable at the axon initial segment. Despite a severe reduction in channel activity, the lifespan and motor function of Scn8a(9J/9J) mice are significantly better than null mutants lacking channel protein. The clinical phenotype of this severe hypomorphic mutant expands the spectrum of Scn8a disease to include a recessively inherited, chronic and progressive movement disorder.