Extensive variations exist in the heavy and light chain components of myosin in vertebrate striated muscles. In the present study, we have characterized a specific contractile property, velocity of shortening, and protein subunit composition of single fibers from adult rabbit soleus muscles. Maximum velocity of shortening (Vmax) was measured using the slack test method, and the myosin composition of these same fibers was determined using an ultrasensitive sodium dodecyl sulfate-polyacrylamide gel electrophoresis system. While most fibers were found to have velocities between 0.5 and 1.0 muscle length/s, several had velocities distributed between 1.33 and 2.99 muscle length/s. The fibers in the slower group had myosin subunits that were solely of the slow type; however, those in the faster group contained both fast and slow heavy chains and light chains. The velocity of shortening measured in fibers having both myosin types was highly correlated with the myosin heavy chain composition, with velocity increasing as the proportion of fast-type heavy chain increased. Variations in light chain composition, particularly fast and slow myosin light chain 1, appeared to occur independently of the variations in heavy chain composition, suggesting that some myosin molecules consist of mixtures of slow- and fast-type subunits.