Two myosin isoforms are expressed in myocardium; homodimers (V₁) and ßß homodimers (V₃). V₁ exhibits higher velocities and myofibrillar ATPase activities compared to V₃. We, too, observed this for cardiac myosin from normal (V₁) and propylthiouracil-treated mice (V₃). Actin velocity in a motility assay (V_actin) over V₁ myosin was twice that of V₃ as was the myofibrillar ATPase. Myosin's average force (F_avg) was similar for V₁ and V₃. Comparing V_actin and F_avg across species for both V₁ and V₃, mouse V₁ has greater V_actin and F_avg compared to rabbit V₁ (47). Mouse V₃ V_actin was twice that of rabbit V_actin. To understand myosin's molecular structure and function, we compared - and ß-cardiac myosin sequences from rodents and rabbits. The rabbit - and ß-cardiac differed by 8 and 4 amino acids, respectively, when compared to rodents. These residues are localized to both the motor domain and rod. These differences in sequence and mechanical performance may be an evolutionary attempt to match a myosin's mechanical behavior to the heart's power requirements.