Myotonia congenita is an inherited disorder of sarcolemmal excitation leading to delayed relaxation of skeletal muscle following contractions. Mutations in a skeletal muscle voltage-dependent chloride channel, CIC-1, have been identified as the molecular genetic basis for the syndrome in humans, and in two well characterized animal models of the disease: the myotonic goat, and the arrested development of righting (adr) mouse. We now report the molecular genetic and electrophysiological characterization of a canine CIC-1 mutation that causes autosomal recessive myotonia congenita in miniature Schnauzers. The mutation results in replacement of a threonine residue in the D5 transmembrane segment with methionine. Functional characterization of the mutation introduced into a recombinant CIC-1 and heterologously expressed in a cultured mammalian cell line demonstrates a profound effect on the voltage-dependence of activation such that mutant channels have a greatly reduced open probability at voltages near the resting membrane potential of skeletal muscle. The degree of this dysfunction is greatly diminished when heterodimeric channels containing a wild-type and mutant subunit are expressed together as a covalent concatemer strongly supporting the observed recessive inheritance in affected dog pedigrees. Genetic and electrophysiological characterization of the myotonic dog provides a new and potentially valuable animal model of an inherited skeletal muscle disease that has advantages over existing models of myotonia congenita.