1. Clam-foot paramyosin strongly inhibited superprecipitation of clam-foot actomyosin whereas it did not at all affect its ATPase reaction. On the other hand, superprecipitation of rabbit actomyosin and its ATPase reaction were both inhibited by clam foot paramyosin. 2. Clam-foot paramyosin did not at all affect the Mg-ATPase activity of clam-foot myosin alone (with no actin) but inhibited that of rabbit skeletal myosin alone. 3. Clam-foot paramyosin did not affect the ATPase reaction of rabbit acto-clam myosin but inhibited its superprecipitation. In other words, the effect of paramyosin on the ATPase reaction of actomyosin was simply dependent on the myosin source. 4. Clam-foot tropomyosin slightly enhanced superprecipitation of clam-foot actomyosin and scarcely affected its ATPase reaction, but it inhibited the reactions both of skeletal actomyosin and of rabbit acto-clam myosin. 5. Rabbit skeletal light-meromyosin (LMM) inhibited the ATPase and superprecipitation reactions of rabbit skeletal actomyosin, but it scarcely affected the ATPase reaction of rabbit acto-clam myosin while inhibiting its superprecipitation. In other words, rabbit LMM behaved the same way as clam paramyosin did. 6. Speculating on these findings and our previous findings, we suggest that the formation of a certain filamentous structure is the essential link between the ATPase reaction and contraction in molluscan muscles.