The invention relates to pre-expanded plastic beads based on poly-para-methylstyrene with the following properties:(a) the bead size lies between 0.2 and 3 cm,(b) the thermal stability, measured in accordance with DIN 53,424 is above 110.degree. C., and(c) the bulk density is below 35 g/l.The corresponding expandable plastic beads are prepared through the polymerization of para-methylstyrene in an aqueous suspension with the addition of volatile organic blowing agents during or after polymerization, whereby polymerization occurs in the presence of molecular-weight-regulating substances. Typical molecular-weight-regulating substances are organic chain transfer agents with a transfer constant K between 0.1 and 50, said agents being added in amounts ranging from 0.1 percent by weight to 1 percent by weight when a yield ranging from 20 to 95 percent is reached. Oligomers of the styrene or para-methylstyrene in quantities from 0.1 percent by weight to 10 percent by weight may also be employed.