Plastic deformation in spherulitic polypropylene includes a component of dilatational strain. Residual volume changes have been measured as a function of uniaxial strain for tension, compression and cyclic tests. In compression, the volume changes were measured during the test while the specimen was under had and the stress maximum was found to be related to the onset of rapid dilation. The dilation for all modes of mechanical testing was found to be linearly dependent on the tensile component of the strain. Microstructural changes responsible for these observations were examined using transmission electron microscopy of permanganic etched interior surfaces of the deformed specimens. Microcrazes along interlamellar planes were found in all deformed specimens. Fatigue failure in symmetric tension/compression tests occurred by accumulation of crazes, predominantly on the tensile half cycles.