Type 1 diabetes is a multigenic autoimmune disease, the genetic basis for which is perhaps best characterized in the nonobese diabetic (NOD) mouse model. We previously located a NOD diabetes susceptibility locus, designated Idd11, on mouse Chromosome (Chr) 4 by analyzing diabetic backcross mice produced after crossing NOD/Lt with the nondiabetic resistant strain C57BL/6 (B6) strain. In order to confirm Idd11 and further refine its location, three NOD congenic mouse strains with different B6 derived intervals within Chr 4 were generated. Two of the congenic strains had a significant decrease in the cumulative incidence of diabetes compared with NOD/Lt control mice. The third NOD congenic strain, containing a B6 interval surrounding the Slc9a1 locus, was not protected against diabetes. These results define a new distal boundary for Idd11 and eliminate the Slc9a1 gene as a candidate. The Idd11 locus has now been definitively mapped to a 13cM interval on mouse Chr 4.