Laboratory deformation experiments were carried out on two single-crystal clinopyroxenes: chrome diopside and hedenbergite. Tests were made in a Griggs solid medium, triaxial, hot creep tester. A confining pressure up 1000 MPa was applied in all experiments. Crystals were deformed at strain rates from 10-4 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), <001> and (001), <100> mechanical twinning systems in clinopyroxene to a high resolved shear stress in the sense appropriate for mechanical twinning to occur. The second orientation subjects these systems to an equal resolved shear stress but in the opposite sense so that mechanical twinning is not possible. Effects of temperature and strain rate on the flow stress are observed for the two clinopyroxene compositions. Mechanical twinning on the system (100), <001> is observed to be the primary deformation mechanism in crystals oriented favorably for twinning. The resolved shear stress required for mechanical twinning on this system is 140 MPa for hedenbergite and 100 MPa for chrome diopside. The smaller twinning stresses for diopside are considered to be due to abundant inclusions in the crystals. The mechanical twinning stress is nearly independent of temperature from 400¿C to 850¿C and strain rate from 10-4 to 10-8 s-1. Above 850¿--1000¿C for diopside and 1000¿C for hedenbergite, deformation occurs by dislocation glide and is strongly temperature and strain rate dependent. Crystals tested in the orientation for which mechanical twinning is not possible deform by two different dislocation glide mechanisms depending on temperature. Details of these experiments are reported in part 2. Results for mechanical twinning predict that this is not an important deformation mechanism in the earth unless resolved shear stress on the twinning system exceeds 140 MPa. Lack of dependence of twinning stress on strain rate implied that large, rapid strains will occur in this mineral if the twinning stress is reached. Independence of twinning stress in clinopyroxenes from temperature, strain rate, or composition allows use of this mineral as a geopiezometer. At plate margins where stresses are high, presence of mechanically twinned clinopyroxene grains allows determination of the orientation and magnitude of stresses responsible for the deformation.