Tungsten trioxide nanowires of a high aspect ratio have been synthesized in situ in a TEM under an electron beam of current density 14A/cm(2) due to a massive polymorphic reaction. Sol-gel-processed cubic phase nanocrystals of tungsten trioxide were seen to rapidly transform to one-dimensional monoclinic phase configurations, and this reaction was independent of the substrate on which the material was deposited. The mechanism of the self-catalyzed polymorphic transition and accompanying radical shape change is a typical characteristic of metastable to stable phase transformations in nanostructured polymorphic metal oxides. The findings are important to the controlled electron beam deposition of nanowires for functional applications starting from colloidal precursors.