The viscoelastic properties of polycrystalline solid methane and carbon dioxide were measured in a low-temperature deformation apparatus. It was found that solid methane flowed easily under a shear stress of 5 MPa even at a temperature of 77 K. Its stress was relaxed with exponent 1.9¿0.2 in non-Newtonian power-law. The effective viscosity of solid methane estimated at 0.1 MPa was 1011 Pa sec. Solid carbon dioxide was deformed brittly at 77 K.¿ 1997 American Geophysical Union