Muscle fiber diameter and fiber-type distribution were analyzed during callus distraction. The right tibia in 24 beagles was lengthened 2.5 cm by callus distraction after osteotomy and application of a ring fixator. Distraction was started at the fifth postoperative day, at a rate of two times for 0.5 mm per day. Twelve dogs that underwent limb-lengthening and three dogs in the control group that did not undergo limb-lengthening were killed at the end of the 25-day distraction phase (group A). The remaining dogs (12 that underwent limb-lengthening and three that did not) were killed after an additional consolidation period of 25 days (group B). The tibialis anterior, extensor digitorum longus, peroneus longus, and gastrocnemius muscles were removed from the right limb (which had undergone distraction) and the left control side of each animal. Crosscut cryostat sections were stained by adenosine triphosphatase at pH 4.3 and 9.4 to determine the size and distribution of types I and II fibers. Morphometric analysis of the muscle fibers was performed by a computer-assisted two-point technique. On the lengthened side, the muscles revealed marked atrophy affecting predominantly type-II fiber in the dogs in group A and affecting both fiber types in dogs in group B. Fiber density increased in both groups. In addition, fiber-type grouping indicative of reinnervation was obvious in group B. Fiber-type distribution in the dogs in group B showed a shift toward type I in the tibialis anterior (p = 0.043) and extensor digitorum longus (p = 0.034) muscles and a shift toward type II in the gastrocnemius (p = 0.038). The data show that tension-stress during tibial lengthening leads to atrophy of type-II fiber, reflecting disuse of muscle fiber in the distraction period as well as neurogenic atrophy followed by the reinnervation processes. Furthermore, the data are consistent with the occurrence of histoneogenesis during limb-lengthening resulting in an increase in fiber density.