AMORPHOUS alloys formed by rapid solidification of a metallic melt are of considerable technological interest as high-strength materials1-8. As they are not in thermodynamic equilibrium, these materials tend to crystallize on heating9,10. A high degree of crystallization leads to embrittlement, but if it can be arrested when the crystallites are of only nanometre dimensions, the resulting amorphous-nanocrystalline composite actually has greater strength than the original amorphous material11. There is consequently much interest in understanding the mechanisms of crystallization. Previous studies have suggested that mechanical deformation can induce crystallization12-16. Here we report the direct observation of crystallization within the shear bands of aluminium-based amorphous alloys induced by bending. The crystals are face-centred cubic aluminium, 7-10 nm in diameter, and seem to form as a consequence of local atomic rearrangements in regions of high plastic strain. We suggest that mechanical deformation might therefore be used to form high-strength amorphous-nanocrystalline composites.