High-strain-rate superplastic magnesium alloy, AZ91, was processed through the ingot metallurgy route. The relationship between working temperature and resulting grain size indicated that the grain size tends to decrease with decreasing working temperature in AZ91. Based on this preliminary result, the ingot was hot extruded at a relatively low temperature of 523 K with a reduction ratio of 44. A very fine grain size of 1.7 \\micron was attained only by hot extrusion. The fine-grained structure was stable blow 573 K. Owing to the fine grain size, high-strain-rate superplasticity was observed at temperatures of ∼548 K.