Knoop and Vickers hardness measurements have been made on tantalum carbide and hafnium carbide single crystals. The hardness varies with orientation of the indenter in the crystal, but indentations in the two carbides are of very different character, TaC behaves in a relatively ductile manner and deforms plastically before cracking, while HfC exhibits extremely limited plastic flow and cracks on indentation. Moreover, the preferred slip plane is 111 in TaC but is 110 in HfC. These results are related to the reported physical properties of these carbides. In particular, the observed mechanical behaviour of TaC appears to be consistent with the more metallic nature of this carbide.