The actin binding and ATPase kinetics of cardiac myosin subfragment-1 were compared with prior studies on skeletal myosin subfragments. Previous kinetic studies on rabbit skeletal subfragment-1 (S-l) have revealed two important features of the actomyosin ATPase activity. First, hydrolysis of ATP by myosin subfragment-1 proceeds both when S-l is bound to actin and when it is dissociated from actin. Second, the actin concentration required to reach half the maximum ATPase activity, Ka(ATPase), is considerably lower than the actin concentration required to bind half the subfragment-1 during steady state hydrolysis of ATP, Ka(binding). These kinetic facts require that skeletal myosin hydrolyze ATP without dissociating from actin; therefore, a “nondissociating” pathway for ATP hydrolysis exists. The studies reported here show that porcine cardiac S-l is very similar to rabbit skeletal S-l. Under identical conditions to prior work on skeletal S-l, the Ka(ATPase) of porcine cardiac S-l is approximately equal to that reported for skeletal S-l. This is also true for Ka(binding). Comparison of Ka(ATPase) and Ka(binding) shows that for cardiac proteins Ka(ATPase) is fourfold to sixfold stronger than Ka(binding), i.e., half maximal ATPase activity is achieved at about one fifth the actin necessary to reach 50% binding. The extrapolated maximum ATPase activity at saturating actin concentration for cardiac S-l is consistently slower than skeletal S-l by about a factor of 2.5. Furthermore, studies of the actoS-1 ATPase activity at high actin concentrations as well as with crosslinked actoS-1 show no significant inhibition, implying the requirement of a “nondissociating” pathway for ATP hydrolysis by cardiac myosin subfragment-1.