Tensile tests were made on specimens of commercial Ta of different grain-sizes, and the variation of lower yield stress with grain-size 2d was used to assess the effect of temperature (23, -78 and -196 deg C) and strain-rate (0.0019 and 0.0996 sec. at each temperature) on the parameters sigma sub i and k sub y in a Petch type equation. The results indicate that the sensitivity of the yield strength to temperature and strain-rate arises almost entirely from the effects that these variables produce on sigma sub i. The high resistance of Ta to brittleness has been confirmed, all of the specimens showing pronounced necking and predominantly fibrous fractures; on the fracture surfaces of the coarser grained specimens tested at -196 deg C (both strain rates) some cleavage facets were observed. The greater ductility of Ta relative to Nb, Mo, and mild steel is attributed to a low value of k sub y and a high value of shear modulus.