Semicrystalline polymers exhibit different levels of microstructure: macromolecules, individual crystalline lamellae, stacks of lamellae separated by amorphous layers, spherulites. Due to such complexity, the understanding of their macroscopic mechanical behaviour requires study of the plastic deformation mechanisms which occur from the nanoscopic to the microscopic scale. We are interested here in the local plastic events which are responsible for the yielding of semicrystalline polymers. Our aim is to show that atomic force microscopy (AFM) is a good tool to image and characterise those mechanisms down to the nanoscopic scale. Polyamide 6 films have been drawn just beyond the yield stress. AFM observations show that, at temperatures T < 160 degrees C, the plastic deformation results mainly from the initiation of shear bands and their propagation through lamellae, spherulites and the whole sample. In contrast, at T > 160 degrees C, AFM images exhibit spherulites which deform plastically by developing a fibrillar structure in their central part while their polar zones are undeformed.