This study focuses the aging-related modification of skeletal fiber types in two skeletal muscles of different-age swine (6 and 18 month). Rectus abdominis and vastus medialis were employed. It was performed an immunohistochemical staining for slow fibers and it was made a quantitative evaluation, using an automatic interactive image analysis system. The percentage of slow fibers decreased in adult swine. Moreover, slow fibers in rectus abdominis were less numerous than in vastus medialis. Aging and muscle function are two important factors able to modify fiber types. Morphometric analyses can ascertain this modification for diagnostic or nourishmental purposes.