Laboratory deformation experiments were carried out on two single-crystal clinopyroxenes: chrome diopside and hedenbergite. The tests were made in a Griggs solid-medium, triaxial, not creep tester. A confining pressure of 1000 MPa was applied in all the experiments. The crystals were deformed at strain rates from 10-4 s-1 to 10-8 s-1 and at temperatures from 400 ¿C to 1200 ¿C. Two orientations of the crystals with respect to the maximum principal compressive stress were tested. The first orientation subjects the (100), <100> and (001), <100> mechanical twinning systems in clinopyroxene to a high, resolved shear stress appropriate for mechanical twinning to occur. The second orientation subjects these systems to an equal resolved shear stress, but in a way that mechanical twinning is not possible. The effects of temperature and strain rate on the flow stress were observed for the two clinopyroxene compositions. Mechanical twinning on the system (100), <001> was observed to be the primary deformation mechanism in crystals oriented favorably for twinning. Above 850 ¿C, deformation occurs by dislocation glide and is strongly temperature and strain-rate dependent. In the orientation for which mechanial twinning is not possible, crystals deformed by two different dislocation glide mechanisms, depending on temperautre. From 400 ¿C to 900 ¿C, deformation in hedenbergite occurs by dislocation glide controlled by lattice resistance. This thermally activated mechanism has an activation energy of 285 kJ/mol (68 kcal/mol) and an activation area that expresses the effect of the applied stress on the total activation energy. This mechanism only operates at stresses above 520 MPa. At temperatures above 900 ¿C and applied stresses less than 520 MPa, flow in hedenbergite occurs by climb-controlled dislocation glide. This leads to a thermally activated power law flow equation. The activation energy is 523 kJ/mol (125 kcal/mol). Strain rate depends on the stress raised to the power 3.6. The flow law derived for hedenbergite at high temperatures and low stresses may be extrapolated to a strain rate of 10-14s-1, thought to be appropriate for flow in the mantle. This flow law predicts an equivalent viscosity of hedenbergite of 1021 poise at 800 ¿C and 1019 poise at 1200 ¿C.