We investigated whether a relationship exists between the fiber type distribution in skeletal muscles and the insulin resistance of different ages of diabetic rats. The fiber type distributions of the slow soleus and fast plantaris muscles in 5-, 9-, and 21-week-old male Otsuka Long-Evans Tokushima Fatty (OLETF) rats as an animal model of spontaneous type 2 diabetes mellitus were examined and compared with those in age-matched non-diabetic Long-Evans Tokushima Otsuka (LETO) rats. In the soleus muscle, a higher percentage of type I fibers was observed in the 9- and 21-week-old OLETF rats compared with the age-matched LETO rats, and there were no type IIA fibers remaining in the soleus muscle of the OLETF rats. In the plantaris muscle, a lower percentage of type IIA fibers and a higher percentage of type IIB fibers were observed in the 9- and 21-week-old OLETF rats compared with the age-matched LETO rats. In addition, a lower percentage of type I fibers in the plantaris muscle was observed in the 9-week-old OLETF rats compared with the age-matched LETO rats, and there were no type I fibers in the plantaris muscle of the 21-week-old OLETF rats. All 20-week-old OLETF rats showed serum insulin concentrations of more than 20 ng/ml. The present study suggests that insulin resistance and impaired glucose tolerance from altered muscle fiber type are linked to the cause of diabetes.