To help answer the question of how much solid CO2 exists in the Martian south polar cap, we performed a series of laboratory triaxial deformation experiments at constant displacement rate in compression on jacketed cylinders of pure, polycrystalline CO2. Test conditions were temperatures 150K, hydrostatic confining pressures 5≤P≤40 MPa, and strain rates 4.5¿10-8≤&egr;˙≤4.3¿10-4 s-1. Most of the measurements follow a constitutive law of the form &egr;˙=A&sgr;n exp(-Q/RT), where &sgr; is the applied differential stress, R is the gas constant, and the other constants have values as follows: A=103.86 MPa-ns-1, n=5.6, and Q=33 kJ/mol. Solid CO2 is markedly weaker than water ice. Our results suggest that the south polar cap on Mars is unlikely to be predominately solid CO2, because the elevation and estimated age of the cap is difficult to reconcile with the very weak rheology of the material. ¿ 1999 American Geophysical Union