A multi-filament superconducting wire in which the filaments comprise zirconia-stabilized ultra-fine grain Nb.sub.3Sn. The superconducting wire is formed by wire-drawing a preform comprising a metallic matrix and at least one niobium alloy rod having zirconium and oxygen in solid solution and heat treating the drawn wire in the presence of tin to yield at least one continuous filament comprising ultra-fine grain Nb.sub.3Sn having semi-coherent ZrO.sub.2 precipitates disposed therein. The ZrO.sub.2 precipitates serve to stabilize the ultra-fine grain microstructure of the Nb.sub.3Sn at temperatures up to 1100.degree. C. and allows Nb.sub.3Sn to maintain the ultra-fine grain microstructure when heat treated at temperatures that are greater than those previously used. By using higher temperatures to form Nb.sub.3Sn, the time required for heat treatment can be significantly reduced.