(1) The tension and shortening relation was investigated for a single fiber of the sartorius muscle of the toad by means of RCA 5734 and the phototube mechanograph.(2) When the initial length is not changed and the equivalent mass is adjusted accordng to the cross section of the muscle, the amount of the initial shortening of a single fiber is found to be the same as that of the whole muscle.(3) A dual hypothesis was advanced to account for all the observed results from the whole muscle and single fiber. The hypothesis is based on the assumption of the existence of the rapid and slow element in the fiber.