The flow properties of β-Zr-Nb (Cb) alloys were investigated by means of compression testing in the strain rate range 10-1 to 10-5 s-1 and from 725 to 1025°C. The flow curves obtained on Zr-Nb alloys containing 10, 15 and 20 pct Nb exhibited flow softening, and the magnitude of this effect decreased as the temperature was increased. All three alloys also exhibited anneal hardening, i.e. an increase in flow stress at 825°C with annealing time at 1000°C. Neither the flow softening, nor the anneal hardening could be associated with environmental effects, as in Zr-Mo alloys, nor could they be attributed to texture changes or to the occurrence of dynamic recrystallization. On the basis of X-ray and microprobe investigations, as well as grain size measurements, it is concluded that the anneal hardening is due to the combined effect of grain growth and the formation of solute clusters during annealing. The occurrence of flow softening is attributed to the destruction of the solute clusters by straining. Stress-strain curves were also determined for Zr-2.5 pct Nb. Unlike the high Nb alloys, these materials exhibited neither flow softening nor anneal hardening. The flow stresses were found to be highly strain rate dependent, with stress sensitivities of about 5.5 for yielding and 4.5 for steady state flow.