Jacketed cylindrical specimens of Yule marble and calcite single crystals have been twisted at a constant rate of 0.1 radian per minute under confining pressures of 1000 to 2750 kg/cm2, at temperatures from 24°C. to 300°C., and under axial loads of 0 to 1000 kg/cm2. Permanent twists of as much as 1.6 rad/cm have been obtained. The yield stress and ductility of the marble increase with confining pressure and are further enhanced by an axial compressive load. The yield stress decreases, and the ductility increases as the temperature is raised. Torsion under confining pressure alone leads to tensile fracture across a helical surface which is everywhere normal to the least principal stress, a tension of about 200 kg/cm2. In specimens under an axial compressive load, failure is by shear fracturing approximately parallel to the plane of maximum shearing stress. Petrographic studies reveal that macroscopically the shear occurs parallel to circular sections. The gliding elements in the grains of the aggregate and in the single crystals have not yet been delineated, but the evidence suggests translation gliding on certain planes not previously observed in marble deformed under uniaxial compression or extension. Mechanical twinning on e{011̄2} is common but cannot be the predominant flow mechanism.