Cavitation during uniaxial deformation of isotactic polypropylene (PP) was studied in samples having different amounts of a and beta crystals. It has been shown that deformation is always accompanied by neck formation. Because the very beginning the plastic deformation is accompanied by cavitation. The scale of cavitation, determined with the use of X-ray scattering and volume measurements increases with an increase of the content of beta phase in the examined samples. Cavities change their shape with deformationat first they are elongated perpendicularly to the direction of stretching, next, at the strain of 1.5, they reorient in the direction of deformation, which is connected with changes in the surrounding lamellar structure. The calculations of Guinier's radius showed that two populations of voids are present in deformed PP samples, characterized by the gyration radii of 5 and 13 nm for small deformations. Estimations based on the volume increase and sizes of cavities indicate that typically more than one cavity are present in amorphous layers adhering to lamella surfaces. (C) 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012