Transmission electron microscopy was used for in-situ observations of transformations in single-crystal (100) sphalerite CdS films to the wurtzite phase. The nucleation was induced by a pulse of a concentrated electron beam, and the lateral growth by partial concentration of the electron beam and/or by general heating of the film by means of wire resistance. A relation was found between the defect nature of transformed wurtzite areas and their relative orientation to the original sphalerite lattice. A discussion of the transformation mechanisms is given.