Tensile specimens of superplastic forming grade IN718 superalloy, containing banded microstructure in the as received state, were deformed at high temperatures T to investigate the stress σ versus strain rate ε behaviour, the nature of the stress versus strain ε curves, ductility, and microstructure upon failure. The log σ-log ε plot for the range ∼5 × 10-6-3 × 10-2 s-1 at T=1173-1248 K exhibited a strain rate sensitivity index m = 0·62 at low strain rates and m = 0·26 at high strain rates, representing region II and III behaviour, respectively. The activation energies were estimated to be 308 and 353 kJ mol-1, respectively. All the σ-ε curves, obtained at ε = 1 × 10-4s-1 for the temperature range 1173-1273 K, and at T= 1198 K for the strain rate range 1 × 10-4-1 × 10-2s-1, exhibited initial flow hardening, followed by flow softening. The microstructures revealed dynamic recrystallisation, grain growth, cavitation, and a variation in the amount of second phase particles. Grain growth and cavitation were found to increase with temperature in region II. Excessive grain growth at 1273 K led to the elimination of region II. Grain growth and cavitation were both found to be less pronounced as the strain rate increased in region III. © 2000 IoM Communications Ltd.