Abstract Crystallization kinetics of polymers crystallizing from the melt are influenced by the mechanical history of the sample. Strain free samples show a larger crystallization half time than samples in which the strain has not been completely removed by relaxation at a suitable temperature. A larger number of nuclei has been assumed to be present in strained samples than in strain free ones. This has been called “orientation induced” or “stress induced” nucleation. This communication presents a morphological description of the phenomenon and of the underlying type of nucleation. My observations were made with a microscope (Reichert ‘Zetopan-Pol’) equipped with a hot stage and 24-mm camera, and mainly with the polarizers crossed. For most of my experiments I used polypropylene, but similar phenomena were also observed with several other polymers.