Background: Nitric oxide (NO) has long been recognized to affect muscle contraction , both through activation of guanylyl cyclase and through modification of cysteines in proteins to yield S-nitrosothiols. While NO affects the contractile apparatus directly, the identities of the target myofibrillar proteins remain unknown. Here we report that nitrogen oxides directly regulate striated muscle myosins. Principal Findings: Exposure of skeletal and cardiac myosins to physiological concentrations of nitrogen oxides, including the endogenous nitrosothiol S-nitroso-L-cysteine, reduced the velocity of actin filaments over myosin in a dose-dependent and oxygen-dependent manner, caused a doubling of force as measured in a laser trap transducer, and caused Snitrosylation of cysteines in the myosin heavy chain. These biomechanical effects were not observed in response to Snitroso-D-cysteine, demonstrating specificity for the naturally occurring isomer. Both myosin heavy chain isoforms in rats and cardiac myosin heavy chain from human were S-nitrosylated in vivo. Significance: These data show that nitrosylation signaling acts as a molecular "gear shift" for myosin-an altogether novel mechanism by which striated muscle and cellular biomechanics may be regulated.