In a process for preparing expanded or expandable polymer beads based on biodegradable saturated polyesters made from at least one diol and from at least one dicarboxylic acid, A. polymer pellets are dispersed in a liquid, B. a volatile blowing agent is injected into the dispersion and made to impregnate the pellets at an elevated temperature, C. the pressure on the dispersion is reduced at an elevated temperature, whereupon the pellets comprising blowing agent expand to give foam, and, respectively, D. the pellets are isolated from the liquid.