In many industrial fields, the development of porous and lightweight polymer composite structures is of great interest. These structures may have several advantages compared to a massive solid of similar chemical nature, such as better mechanical properties, cushioning, insulation or sound and heat absorption. Three different methods are developed for the foaming of materials: batch, injection and extrusion foaming. These techniques employ highly toxic and harmful substances such as azodicarbonamide as blowing agents, which has increased the use of supercritical fluids as blowing agents, sc-CO2 being the most employed. In addition, petroleum-based thermoplastics are widely used as polymer matrices, but, due to the shortage of fossil resources and the rise of environmental concerns, biopolymers (bio-based and/or bio-degradable, polymers) are more and more used. In this context, the foaming of biopolymers using supercritical CO2 as a blowing agent has become a subject of interest. For this work, polylactic acid (PLA) is employed as the polymer matrix. PLA’s interesting properties have made it the most studied biopolymer, and consequently generated a great amount of interest in different industrial fields. The foaming of PLA biocomposites has been investigated before [1-2], but the effect of fillers on the cellular morphology remains a misunderstood and poorly studied subject. This work aims to explain the effects of size and aspect ratio of a filler as well as its content on the characteristics of PLA foams obtained by supercritical CO2-assisted extrusion process. Short (S) and long (L) cellulose fibres with aspect ratios (lenght/diameter) of 1.5 and 5-7 respectively, have been used as fillers. Fibres were compounded with the polymer through hot melt extrusion at different volume fractions (4 and 12 vol.%) (mass fractions 5 and 15 wt.% respectively). During the extrusion foaming process, two different volumetric flow rates of CO2 were studied (2.5 and 3 mL min-1), and the effect of die temperature was evaluated as well. Techniques as gas pycnometry, scanning electron microscopy (SEM) and modulated differential scanning calorimetry (MDSC) have been used to identify the characteristics of the produced foams.