The relationship between the predominance of fast and slow muscle fibers of the vastus lateralis and "in vivo" torque velocity properties in 22 female athletes was studied. Fiber types were classified according to the histochemical myofibrillar adenosine triphosphatase technique at a basic pH. Maximal extensor troques were recorded at 30 degrees from full extension at four selected velocities. While results confirm earlier reports on muscle fiber type and performance, an additional finding was that as knee extension velocities increased from 0 to 95 degrees/s angle specific extensor torque production did not decline as seen in in vitro muscle preparations. The difference in extensor torque between 0 and 96 degrees/s appeared far more critical than the differences observed between 96 and 288 degrees/s. Significant differences in torque were seen at 96, 192, and 288 degrees/s in thos with greater than 50% and less than 50% slow-twitch fibers. When expressed per kilogram of body weight the subjects with greater than 50% fast-twitch fiber produced the greatest torque at 192 degrees/s. These results suggest that the velocity at which torque begins to decline in vivo is related to the proportion of slow-twitch fibers in the vastus lateralismuscle.