Natural chalcopyrite from two localities was experimentally deformed in compression under 1.5 kilobars of confining pressure at temperatures up to 300 degrees C over the strain rate range 10 (super -2) to 10 (super -6) sec (super -1) . At 25 degrees and 100 degrees C the stress-strain behavior was relatively independent of temperature and strain rate and the deformation proceeded predominantly by intragranular and grain-boundary fracturing (microfracturing). From 150 degrees to 300 degrees C strength decreased with increasing temperature and decreasing strain rate, and the dominant mode was intragranular slip and deformation twinning. The higher temperature behavior of the two different experimental materials was similar in spite of differences in impurity and grain size. The low-temperature microfracturing strength, however, was higher in the finer grained, less pure chalcopyrite.Extrapolation of the experimental data for "steady state" plastic flow to representative geological strain rates indicates stress differences in the order of 250 to 1,000 bars at 100 degrees C, 80 to 500 bars at 200 degrees C, and 30 to 300 bars at 300 degrees C. These values are not expected to vary with confining pressure. Syntectonic recrystallization might lower these stress differences even further.Chalcopyrite is expected to flow plastically in the crust except for very shallow conditions where low confining pressures and temperatures and relatively high stress differences prevail.In etched polished sections evidence of naturally deformed chalcopyrite can be provided by slip lines, deformation twins, subgrains, high-energy serrated grain boundaries, distorted twin boundaries, and elongation (straining) of grains. Such evidence may be partially or wholly obliterated by subsequent recrystallization.