Abstract: The production of polymer beads by a suspension polymerization in which a vinyl aromatic monomer having a free-radical generating catalyst dissolved therein is suspended in an aqueous medium with the aid of from 0.2 to 1.0 per cent by weight, based on monomer, of a finely divided phosphate suspending agent and heated to cause the monomer to polymerize into polymer beads is improved by treated the aqueous medium containing the phosphate and any extender with ultrasound prior to adding the monomer and polymerizing. The resulting polymer bead size is thereby found to be smaller than would have been obtained without the use of ultrasound.