The novel diabetic mouse model Munich Ins2^C95S was discovered within the Munich N-ethyl-N-nitrosourea mouse mutagenesis screen. These mice exhibit a T→A transversion in the insulin 2 (Ins2) gene at nucleotide position 1903 in exon 3, which leads to the amino acid exchange C95S and loss of the A6-A11 intrachain disulfide bond. From 1 month of age onwards, blood glucose levels of heterozygous Munich Ins2^C95S mutant mice were significantly increased compared with controls. The fasted and postprandial serum insulin levels of the heterozygous mutants were indistinguishable from those of wild-type littermates. However, serum insulin levels after glucose challenge, pancreatic insulin content, and homeostasis model assessment (HOMA) β-cell indices of heterozygous mutants were significantly lower than those of wild-type littermates. The initial blood glucose decrease during an insulin tolerance test was lower and HOMA insulin resistance indices were significantly higher in mutant mice, indicating the development of insulin resistance in mutant mice. The total islet volume, the volume density of β-cells in the islets, and the total β-cell volume of heterozygous male mutants was significantly reduced compared with wild-type mice. Electron microscopy of the β-cells of male mutants showed virtually no secretory insulin granules, the endoplasmic reticulum was severely enlarged, and mitochondria appeared swollen. Thus, Munich Ins2^C95S mutant mice are considered a valuable model to study the mechanisms of β-cell dysfunction and death during the development of diabetes.