A fundamental study of the ductile‐brittle behavior of the transition‐metal monocarbides has been initiated through an investigation of the plastic deformation of single crystals of TiC, ZrC, and NbC. A vacuum furnace was used with an Instron testing machine for compression measurements in the temperature range 800° to 1600°C. The specimens were Linde single‐crystal boules, cleaved into bars approximately 1×1×4 mm. The compressive stress was applied longitudinally in a ⟨100⟩ direction in the NaCl structure. The three carbides studied behaved similarly, but NbC was several times harder than TiC and ZrC. The critical resolved shear stress &tgr; decreases exponentially with increasing temperature, increases approximately linearly with increasing carbon content, is little affected by surface condition, and increases as the logarithm of the strain rate. No yield point was observed. The effective stress exponent depends on the strain rate, ranging from 1 to 10. These results are discussed in relation to recent theories of the stress‐strain relation.