To assess myosin heavy chain (MHC) plasticity in aging skeletal muscle with aerobic exercise training, MHC composition was measured at the messenger RNA (mRNA) level and protein level in mixed-muscle homogenates and single myofibers. Muscle samples were obtained from eight nonexercising women (70 ± 2 years) before and after 12 weeks of training (20-45 minutes of cycle exercise per session at 60%-80% heart rate reserve, three to four sessions per week). Training elevated MHC I mRNA (p < .10) and protein (p < .05) in mixed-muscle (54% ± 4% to 61% ± 2%) and single myofibers (42% ± 4% to 52% ± 3%). The increase in MHC I protein was positively correlated (p < .05) with improvements in whole muscle power. Training resulted in a general downregulation of MHC IIa and IIx at the mRNA and protein levels. The training-induced increase in MHC I protein and mRNA demonstrates the maintenance of skeletal muscle plasticity with aging. Furthermore, these data suggest that a shift toward an oxidative MHC phenotype may be beneficial for metabolic and functional health in older individuals.