The ultrastructural characteristics of the inner ear of Jackson shaker mice were analyzed. We used 12 Jackson shaker mutants (js/js) with ages ranging from 10 to 47 days and 10 heterozygotes of the Jackson shaker (js/+) with ages ranging from 10 to 30 days. The most striking findings observed were incomplete differentiation of the stereocilia of the outer hair cells and the maculae, although outer and macular hair cell cytoplasm, including the nerve terminals, became fully developed. Most outer hair cells did not show regular W-shaped configuration of the stereocilia throughout the entire turns of the cochlea except for a few hair cells. In many hair cells of the utricular and saccular maculae, the classical pipe organ configuration of the stereocilia was not observed. The Jackson shaker mice have been reported to have a gene abnormality on chromosome 11, and its gene locus was close to that of our new-mutant mice which showed deranged stereocilia of the outer and macular hair cells. Therefore, future studies can provide additional information on the cytodifferentiation of the stereocilia as a function of the gene on chromosome 11.