The effects of aging on the mechanical properties of myosin were measured in 87 fibers from muscles of humans (n = 40), rats (n = 21), and mice (n = 26) using a single fiber in vitro motility assay. Irrespective of species, an 18-25% aging-related slowing in the speed of actin filaments was observed from 62 single fibers expressing the slow (type I) beta-myosin heavy chain isoform. The mechanisms underlying the aging-related slowing of motility speed remain unknown, but it is suggested that posttranslational modifications of myosin by oxidative stress, glycation, or nitration play an important role. The aging-related slowing in the speed of actin filaments propelled by the type I myosin was confirmed in three mammalian species with an approximately 3,400-fold difference in body size. Motility speed from human myosin was 3-fold slower than from myosin of the approximately 3,400-fold smaller mouse and approximately twofold slower when compared with the approximately 130-fold smaller rat, irrespective of age. A strong correlation was observed between the log values of actin sliding speed and body mass, suggesting that the effects of scaling is, at least in part, due to altered functional properties of the motor protein itself.