Extrusion foaming of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) and two blends of PHBV with cellulose acetate butyrate (CAB) were studied using an azodicarbonamide (AZ) blowing agent and a single-screw extruder. The concentration of the blowing agent was systematically varied from 0 to 4.0 phr to achieve maximum density reduction reaching 41%, as well as to obtain information on the dependence of cell growth on blowing agent concentration. Extruded foams were characterized in terms of their bulk densities and cellular morphologies. Stereological and statistical methods permitted full characterization of the three-dimensional cell size distributions, assessing the average cell diameters (ranging from 58 to 290 mu m) and cell densities (ranging from 650 to 180,000 cm-3). The variation in cellular morphology among foams consisting of different polymer matrix or blowing agent concentration was compared. The results were analyzed by considering the influence of viscoelastic properties of the polymer matrix on the bubble growth during foaming. Significantly higher melt viscosity and elasticity and reduced gas solubility of the PHBV/CAB blends are believed to retard cell coalescence and collapse during foam expansion, resulting in more uniform cell size distribution and better homogeneity of cellular morphology. POLYM. ENG. SCI., 2012. (C) 2012 Society of Plastics Engineers