The behavior of the semicrystalline polymer polypropylene (PP) was studied in the pre-necking domain. Mechanical tests were performed to characterize its viscoelastic and its viscoplastic behavior. The usual approach for studying metals was adopted to investigate the effect of heterogeneities caused by spherulitic morphology. The macroscopic stress was split into an effective stress and a back stress so that the interactions between crystalline lamellae and amorphous phases could be taken into account. An understanding of the relationship between microstructure and the macroscopic strain mechanisms was developed.