Experimentally deformed Maryland diabase has been examined using the transmission electron microscope in order to determine the mechanisms of deformation operative over a range of temperatures and pressures and thereby aid in the interpretation of polyphase flow data. Deformation mechanisms within the plagioclase and clinopyroxene of the diabase have been compared to those within monomineralic aggregates of these phases deformed at the same conditions. Both phases deform by microfracturing, mechanical twinning, and intracrystalline slip. Data for the monomineralic aggregates show that plagioclase is stronger than clinopyroxene at <800° C, but weaker at ≥ 800° C, for a pressure of 15 kbar and strain rate of 10 -6/sec. Observations of the diabase are consistent with this result: at < 800° C, the plagioclase and pyroxene grains are about equally strained, whereas at ≥ 800° C, almost all of the sample strain is accommodated by the plagioclase, the more continuous phase.