Lamellar twinning on (100) and to a lesser extent on (001) are present in clinopyroxenes of deformed mafic and ultramafic rocks. Both sets of twins are mechanical in origin and form where the resolved shear stress on the twin-glide system is high. Statistically, compression axes most favorably oriented for twin gliding in each of a large number of grains are aligned sub-parallel to the direction of maximum compression in experiments. In some deformed diopside-bearing calc-silicate rocks from South Australia, compression and tension axes deduced from twinning are consistent with principal stresses deduced from faulting. It is concluded that lamellar twinning in clinopyroxenes may be used for determination of the direction of principal stresses in natural deformations of rocks.