In the present study we measured, for the first time, the isometric specific force (SF, force normalized to cross sectional area) generated by single intact fibers from fast- (extensor digitorum longus, EDL) and slow-twitch (soleus) muscles from young adult (2-6), middle-aged (12-14) and old (20-24 month-old) mice. SF has also been measured in single intact flexor digitorum brevis fibers from young mice. Muscle fibers have been classified into fast- or slow-twitch based on the contraction kinetics. Maximum SF recorded in EDL and soleus fibers from young and middle-aged mice did not differ significantly. A significant age-dependent decline in maximum SF was recorded in EDL and soleus fibers from young or middle-aged to old mice. The SF was 377 +/- 18, 417 +/- 20 and 279 +/- 18 kPa for EDL fibers from young, middle-aged and old mice, respectively and 397 +/- 17, 405 +/- 24 and 320 +/- 33 kPa for soleus fibers from age-matched mice, respectively. The frequency needed to elicit maximum force in EDL and soleus fibers from middle-aged to old mice did not differ significantly. In conclusion, the specific force developed by both fast and slow-twitch single intact muscle fibers declines with aging and more significantly in the former.