In our laboratory, mice showing signs of osteoarthritic lesions with cinnamon colored (yellowish-brown) hair were discovered in a colony of B6C3F₁ mice. This mouse is characterized by tiptoe walking and swelling and ankylotic changes in the ankle joint. As to radiographic findings, osteoarthritic changes, such as erosion and/or fusion of the bone tissue, were evident in the ankle joints. Histopathological characteristics included irregularity of articular surfaces caused by fissuring and/or erosion with degeneration of articular cartilage, as well as osteophytes with abnormal proliferation of chondrocytes in joint margin regions. Subsequently, ankylotic changes in the ankle joints were completed in the formation of a cartilaginous bridge and fusion of articular cavity with abnormal proliferation of cartilaginous or bone tissues. This mouse strain may provide an additional animal model that is valuable in the study of human osteoarthritis (OA).