A method of extruding a foamed thermoplastic plastics material wherein the plastics material and a pore regulating agent are fed into an extruder and are fused in a fusion zone. The pressure of the fused material is augmented in a further zone and the material is then caused to flow past or through an obstruction so as to be converted into the form of thin layers or strands. Thereafter, steam and a propellant are sequentially injected into the fused mixture in injection zones. The steam is either saturated or superheated and has a temperature not lower than that of the mixture. The pressure of the mixture upstream of the obstruction is higher than the vapor pressure of the steam downstream of the obstruction. The pressure of the fused mixture and steam, while the propellant is being injected, is greater than the condensation pressure of the steam. The mixture is then cooled and discharged in a still further zone whereupon it foams.