The spontaneous phase II to phase I crystal-crystal transformation of polybutene-1 was investigated by electron diffraction and bright- and dark-field imaging of solution and thin-film grown single crystals. Whole single crystals were observed to transform with a single phase II to phase I orientational relationship and, in the case of multiple orientations, the transformed areas were not dependent on growth sectors. These results do not support the views of Holland and Miller of a "twinned" phase II to I transformation determined by growth sectors but are consistent with a transformation scheme introduced by Fujiwara. Nucleation and growth of the transformation are further discussed.