Niobium single crystals of varying orientations have been tested in tension and compression and in alternating tension-compression cycles in the temperature range 77 K-room temperature. Crystals of high purity with centrc-triangle orientations deformed in both tension and compression by slip on the (iOl) plane or on the plane of maximum resolved shear stress (mrss) at temperatures above -200 K and on the anomalous (01 I) plane at low temperatures. There was no detectable Row stress asymmetry for these crystals at room temperature, but deformation in compression became progressively harder than deformation in tension as the temperature was lowered and the assymmetry at 77 K-was about 15% of the actual flow stress. High purity crystals with orientations near [01 I] deformed by ( 211) slip in both tension and compression at room temperature and the flow stress in tension was measurably higher than that in compression. This asymmetry was reversed at lower temperatures and slip traces were then too fine to be visible even under Nomarski contrast but some evidence was found that slip in tension deviated from ( 211) towards (iO1). Some experiments were undertaken also on two less pure crystals with orientations near the [Olll_[ill] edge of the unit triangle. Here coarse (211) slip traces were observed at room temperature and 228 K and the stress in tension was consistently higher than that in compression; catastrophic local flow occurred at 77 K and quantitative measurements could not be made. The results are discussed in terms of existing models of the deformation of b.c.c. metals. CHANG er N/.: DEFORMATION BEHAVIOUR OF NIOBIUM SINGLE CRYSTALS