<PICT:1001740/C3/1> A foamable thermoplastic resin is extruded in the form of a fibre, and the surface only of the fibre is quenched below the foaming temperature while permitting the core of the fibre to expand to a density of 14-45 lbs/cu. ft., thereby forming a fibre having a foamed resin core and an impervious outer skin integrally united with the core, the quenching being such that at least 50% of the diameter of the fibre is foamed core, the balance being skin, integrally united with the core. The fibre is then drawn to orient it longitudinally. The drawing may be accomplished in one or more steps, to a total of 200-2000%, preferably while the fibre is not, e.g. 250-350 DEG F. The fibre is preferably drawn as it leaves the spinneret, so that the core is warm enough to foam while the skin is cool enough to maintain the form of the fibre, but the extruded fibre may also be cooled, wound on rolls and subsequently reheated for drawing. The resin may be polystyrene, polypropylene, polyvinyl chloride, propylene/ethylene co-polymer, vinyl chloride/vinyl acetate copolymer or polyurethanes derived from prepolymers formed from conventional polyols and polyiso-cyanates. Specified prepolymers are made by reacting toluene diiso-cyanate with mixtures of polypropylene glycol with tris-(dipropylene glycol) phosphite, or bis-(dipropylene glycol) hydroxypropoxypropane phosphonate. Other specified polymers are methyl methacrylate, polychlorotrifluoroethylene, styrene / acrylonitrile/ butadiene terpolymer, styrene/butadiene, chlorosulphonated styrene/butadiene, and styrene/divinyl benzene copolymers, and blends of polystyrene with hydrogenated butadiene, and with polyisoprene. The resin may contain an adsorbent having adsorbed thereon a volatile liquid non-reactive with the resin. Specified adsorbents are diatomaceous earth, fuller's earth, silica gel activated molecular sieves, attapulgus clay and activated carbon. The volatile liquid comprising usually 0.1-15% by weight of the polymer may be an aliphatic hydrocarbon, alkanol, ester, aldehyde, ether or halogenated hydrocarbon. Many are specified. The resin composition may also contain a wetting agent and a carbon-dioxide liberating nucleating agent, e.g. a carbonate, bicarbonate or an acid or acid-reacting salt. Many carbonates, bicarbonates and organic acids are specified. In one embodiment (Fig. 1) a foamable polystyrene composition is mixed in 2 and delivered to spinning head 8 at 325 DEG F. and 2500 p.s.i. Fibres 12 are extruded at 300 DEG F. from orifices 10 and are subjected to blasts of air at 200 DEG F. from fans 14 to cool the surface of the fibres to below the foaming temperature, e.g. 220 DEG F. Alternatively, the fibres may be cooled by water, air-water mixture, nitrogen or argon. A continuous and impervious skin 16 is thus formed integral with the foamed core 18 (Fig. 2). The fibres are then drawn between sets of rolls 20, 22 and 24, 26 whereafter they may be wound, or passed through a warming bath 28 and subjected to a second stretching step. Bath 28 contains, e.g. glycerine or Wood's metal. The bath may also be replaced by a hot air oven.