Superplastic behavior was examined in fine-grained (similar to2 mum) WE43 magnesium alloy. that contained, within its grains, spherical precipitates with a diameter of similar to 200 nm. The material exhibited superplasticity with an elongation-to-failure of over 1000% at a temperature of 673 K and a strain rate of 1 x 10(-4) s(-1). Large elongations were obtained in spite of the existence of particles. The dominant deformation mechanism was suggested to be grain boundary sliding accommodated by slip controlled by grain boundary diffusion. Data analysis based on the constitutive equation for superplastic how revealed that the normalized strain rats for particle-strengthened WE43 alloy was about fifty times lower than that for conventional superplastic magnesium alloys. It was suggested that the existence of intragranular particles affects the superplastic flow.