Low-density open-cell foams were obtained by extrusion process from polylactic acid (PLA) and from blends of PLA with thermoplastic starch (TPS) using CO 2 as blowing agent. Two unexpected features were found. First, a 2D cavitation process in the fractured cell walls was unveiled. Elliptical cavities with dimensions in the 100-300 nm range were aligned perpendicular to large cell cracks clearly exhibiting 2D crazing prior to macroscopic cell rupture. Secondly, a significant crystallization rate increase associated with the CO 2 foaming of PLA was discovered. While the PLA used in this study crystallized very slowly in isothermal crystallization, the PLA foams exhibited up to 15% crystallinity providing evidence for CO 2 plasticization effect, and the biaxial stretching upon foam expansion provided conditions that could increase the crystallization rate by several orders of magnitude.