Major contractile proteins were purified from relaxed actomyosin extracted from molluscan catch muscle myofibrils using ammonium sulfate fractionation and divalent cation precipitation. A fraction of this actomyosin was precipitated and purified as a supramolecular complex composed of twitchin (TW), myosin (MY), and myorod (MR). Another TW-MR complex was obtained via the removal of myosin. These supramolecular complexes and filaments assembled from purified myosin contained an endogenous protein kinase that phosphorylated myosin and myorod. Significantly, the activity of this novel myosin-associated (MA) kinase was inhibited at calcium concentrations of >0.1 microM. After partial purification of the kinase, we established that the inhibition resulted from binding of calcium to the substrate (myosin) and not from the binding to the enzyme (kinase). No inhibition was observed when myorod was used as a substrate, although the latter is identical to the rod portion of myosin lacking the head domains. Phosphorylation sites of myorod were identified, three at the C-terminal tip and three at the N-terminal domain. In the presence of calcium, addition of myosin to the TW-MR complex resulted in inhibition of this phosphorylation, while in the absence of myosin, this inhibition was negligible. Added myosin also inhibited phosphorylation of twitchin by PKA-like kinase, the latter also present in the complex. The opposite was true with the TW-MY-MR complex; that is, phosphorylation of myosin was inhibited by twitchin and/or myorod. Thus, in parallel to the well-established direct activation by calcium, molluscan catch muscle myosin also regulated its own phosphorylation. Therefore, in addition to the established phosphorylation of twitchin by PKA-like kinase, phosphorylation of myosin and myorod by myosin-associated kinase appears to play an important role in the development of the catch state.