An Al-6%Mg -0.3%Sc-0.3%Mn alloy was subjected to equal-channel angular extrusion (ECAE) at 325degreesC to a total strain of about 16 that resulted in an average grain size of about 1 mum. Superplastic properties and microstructural evolution of the alloy were studied in tension at strain rates ranging from 1.4x10(-5) to 1.4 s(-1) in the temperature interval from 250 to 500degreesC. It was shown that this alloy exhibits superior superplastic properties in the wide temperature range 250-500degreesC at strain rates over 10(-2)s(-1). The highest elongation to failure of 2000% is attained at a temperature of 450degreesC and an initial strain rate of 5.6x10(-2) s(-1) with the corresponding strain rate sensitivity coefficient of about 0.4. Two different fracture mechanisms were revealed in h, h strain rate superplasticity. At temperatures higher than 300degreesC and/or strain rates less than 10(-1)s(-1), failure took place in brittle manner practically without necking and cavitation played a major role in the failure. In contrast, at low temperatures and/or high strain rates, fracture occurred in a ductile manner suggesting the fracture of samples by localized necking. At these conditions, evidence of very limited cavitation was found in the samples.