Triaxial, compression experiments were conducted to establish the effects of temperature (from 20 degrees to 400 degrees C), confining pressure (from 1 bar to 3 kb), and strain rate (from 10 (super -4) to 10 (super -7) sec (super -1) ) on the mechanical behavior of dry, natural, polycrystalline pyrite of high porosity ( nearly equal 8%). Additional deformation experiments were performed on a similar but less porous ( nearly equal 1%) pyrite aggregate at confining pressures from 1 bar to 2 kb, at a temperature of 20 degrees C and at a strain-rate of 3 X 10 (super -5) sec (super -1) . The porosity of the pyrite aggregates tested by Lang (1968) were determined and used to supplement the results of the present study.Strength depends strongly and directly on confining pressure but is strongly and inversely dependent upon porosity. Strength is independent of strain rate and only slightly dependent upon temperature. Macroscopic ductility is enhanced by increasing temperature, confining pressure, or porosity but is independent of strain rate.When deformed under conditions corresponding to the upper 10 km of the earth's crust, polycrystalline pyrite is much stronger and less ductile than polycrystalline galena, chalcopyrite, or pyrrhotite, or Solnhofen Limestone.The above properties reflect the fact that under the conditions of the present experiments polycrystalline pyrite deforms exclusively by cataclasis.The influence of porosity on the strength of polycrystalline pyrite is related to the energy requirements for fracture. Porosity influences macroscopic ductility by providing local stress relief and by favoring the development of intensely comminuted zones bounding more stable regions.It is suggested that relatively fast laboratory strain rates are adequate to evaluate the mechanical behavior of dry pyrite undergoing cataclastic deformation at much slower geological strain rates and that porosity and fracture density may significantly influence the development of structures in deforming pyrite deposits by facilitating cataclastic "flow".Plastic deformation of pyrite may be possible at high temperatures and high sulfur vapor partial pressures.