A study was done to compare the effects of coverage with skin and those of coverage with a muscle flap on the early return of strength at the site of an osteotomy after interposition of a devascularized segmental autogenous graft, two centimeters long, from a canine tibia. The bone was fixed with a plate. Thirty-two animals were randomized into two groups, one of which was treated with skin coverage and the other of which was treated with muscle flap coverage. Half of the animals from each group were killed at eight weeks and half, at twelve weeks postoperatively. The tibiae were tested to failure in four-point bending. Failure occurred primarily through the site of the distal osteotomy. Maximum bending load (p = 0.0002) and energy absorbed to failure (p less than 0.02) increased significantly between eight and twelve weeks postoperatively in the group in which a muscle flap had been used for coverage. Bending stiffness was significantly greater at eight weeks in the group in which a muscle flap had been used for coverage than in the group in which skin had been used (p less than 0.03). Maximum bending load was also significantly increased at twelve weeks in the group in which a muscle flap had been used compared with the group in which skin had been used (p less than 0.05).