The polymerization of a styrene in water (o/w) emulsion was performed at 30 degrees C (+/-5 degrees C) under ultrasonic irradiation in the absence of any added chemical initiator. The radicals formed as a consequence of the cavitation process were sufficient to cause polymerization. The latices formed in this way were stable bluish-white dispersions. The polymerization rate increased through a maximum at about 30% conversion before decreasing; no region with a steady rate was seen. The final latex diameters were around 50 nm and the polystyrene molecular weights were high (>10(6)), suggesting at most a few polymer chains per particle. Calculations of the particle numbers as a function of monomer conversion suggested that there was a continuous nucleation of the particles. The small particle sizes, high polymerization rates, and continuous nucleation are postulated to be due to the continuous formation of very small oil droplets in the ultrasonic field which can efficiently scavenge the radicals formed during the cavitation process.