Fish myofibrils were prepared for the study on the relation of their ATPase activity to contractile properties, especially in the presence of Mg or Ca ion. The degree of contraction of the myofibrils was measured in relative terms by centrifuging the myofibrillar system at 1, 000rpm for 20minutes, and then measuring the volumes of the fibril layer, in the absence and presence of ATP. The ATPase activity of myofibrils under the same condition as above was also assayed, separately. When Mg ion alone was added as divalent cation to our myofibril preparation, it had little or no effect on the degree of contraction and on the rate of ATP splitting as well. Further addition of EDTA to this Mg system, however, resulted in considerable inhibition of ATPase activity and in the complete inhibition of contraction. On the other hand, Ca ion alone, even at high concentration, somewhat reduced the degree of contraction, although it allowed a very high rate of ATP splitting. These results showed that the fish myofibrillar system closely resembled that of rabbit in its reaction totivards ATP and divalent cations. Three myofibril preparations which consisted of different sized fragments of fibrils were obtained by using differential centrifugation. No differences was found in contractile properties and in their ATPase activity between them.