Cathepsins B and L and cathepsin L-like proteinase degraded myosin heavy chain of actomyosin (AM) into several fragments with molecular masses (MW) of 154, 146, 138, 67, and 36 kDa; 164 and 155 kDa; and 135, 128, and 69 kDa, respectively. In the presence of 0.6 M NaCl, however, the MHC was degraded by cathepsin L into 164, 155, 41, and 37 kDa fragments. The hydrolytic rate of these three proteinases on AM was faster at pH 5.0 than at pH 6.0. Actin seemed to be resistant against hydrolysis by cathepsins B, L, and L-like in the absence of 0.6 M NaCl. According to SDS−PAGE analysis, cathepsin X, a novel cysteine proteinase, did not degrade AM. Keywords: Mackerel; cathepsins; actomyosin; proteolysis; seafood