Myosin was separated from squid brachial muscle by essentially the same method as has been used for squid mantle myosin. Brachial myosin thus partially purified was contaminated with large amounts of paramyosin, which was, in turn, eliminated by the method of KIMURA et al. SDS-polyacrylamide slab gel electrophoresis showed that molecular weights of a heavy chain and two light chains of this myosin are identical to those of corresponding chains of squid mantle myosin. Ca-, Mg- and EDTA-ATPase activities of the brachial myosin were similar to those of the mantle myosin in terms of the dependency on pH, KCl concentration, temperature, and actin activation of Mg-ATPase. The Mg-ATPase activity was as Ca2+-sensitive as that of the mantle myosin, half the maximal activation being obtained at 3 μM Ca. The Ca-sensitivity was lost when 1.5M urea, 0.45M guanidine-HCl, or 3.6% n-butyl alcohol was present in the reaction mixture. Trinitrophenylation deprived the brachial myosin of Ca-senstitvity. These results indicated the identity of both squid myosins in enzymatic properties including Ca-regulatory aspects.