Artificial fibres are made by extruding an acetone solution of a co-polymer of the kind defined below in the form of liquid filaments, setting the filaments by removal of acetone, and collecting the filaments. The co-polymers used are acetone-soluble co-polymers of two or more monomers which are partial substitution derivatives of ethylene in which the sole substituents are chlorine atoms or cyano groups, at least one of said monomers being a di-substitution derivative, and at least one containing an unsubstituted methylene group. Examples of such co-polymers are acetone-soluble co-polymers of vinylidene chloride with acrylonitrile and of vinyl chloride or vinylidene chloride with chloracrylonitrile, methylene malonitrile, maleic dinitrile or fumaric dinitrile and ternary co-polymers of vinylidene chloride and vinyl chloride with any of the cyano derivatives specified. Preferably the co-polymers used contain 50-80 per cent, and especially 55-75 per cent by weight of vinylidene chloride, the remainder being acrylonitrile. Ternary co-polymers of similar constitution except that 20-80 per cent, e.g. 40-60 per cent of the vinylidene chloride is replaced by vinyl chloride are also said to be particularly useful. A suitable co-polymer may be obtained by co-polymerizing vinylidene chloride with acrylonitrile in emulsion, the initial molar ratio of vinylidene chloride to acrylonitrile being between 1 : 3 and 3 : 1, e.g. between 1 : 2 and 2 : 1 and the weight-ratio of the aqueous phase to the initial total monomer being between 5 : 1 and 10 : 1. The filaments formed from the co-polymers may be stretched, e.g. to 200-1000 per cent of their original length, in a heat-softened condition, e.g. during passage through a bath of hot water. In an example an acetone-soluble co-polymer containing 70-75 per cent of vinylidene chloride and the remainder acrylonitrile is made from an emulsion composed of 10 parts of acrylonitrile, 30 of vinylidene chloride, 240 of water, 2 of sodium secondary octadecyl sulphate and 1.2 of potassium persulphate, the polymerization being carried out in a closed autoclave for 16 hours at 40 DEG C., when 15 per cent conversion is obtained. The copolymer is recovered by blowing the latex in the form of fine droplets on to the surface of a bath of ethanol and filtering off the co-polymer thereby precipitated. The co-polymer is washed with water, dried and dissolved in acetone containing 5 per cent of water, to give a 20 per cent solution. The solution is extruded downwards through spinnerets into a vertical spinning cell through which a current of air is drawn upwards. The filaments thus formed are drawn from the bottom of the cell through a short bath of water and then collected on a bobbin. In other examples filaments are made in a similar manner from co-polymers containing 50-60 per cent of vinylidene chloride (the remainder being acrylonitrile) prepared according to examples 3 and 4 of Specification 643,198, and are stretched from 200 to 220 per cent of their original length while passing through a bath of water at 70 DEG C., their tenacity being thereby much increased. In a series of further examples acetone-soluble ternary copolymers containing 20-50 per cent of acrylonitrile, 25-40 per cent of vinylidene chloride and 10-55 per cent of vinyl chloride are prepared by the method of Specification 643,198, from aqueous emulsions of the monomers in which the ratio of water to total monomers is 5 : 1, the catalyst is 0.1 per cent of potassium persulphate and the emulsifying agent 10 per cent of sodium secondary octadecyl sulphate, both calculated on the monomer weight, and the temperature is 50 DEG C. The polymers thus obtained are then spun into filaments and stretched as described above. The filaments may be annealed after stretching by heating them, preferably under slight tension, to a temperature say 20 DEG C. above the maximum to which they will be exposed in use. The heating may be carried out by exposing the filaments to wet steam. The acetone solution of the co-polymer may be wet spun, e.g. by extruding the solution into an aqueous bath containing acetone or another lower aliphatic ketone. Water-soluble inorganic thiocyanates may also be present and the filaments may be stretched to many times their original length in the coagulating bath. The Provisional Specification refers also to the following: (1) the use as emulsifying agents for the monomers in the preparation of the co-polymers of alkali-metal salts of alkyl-aryl sulphonic acids and alkali metal soaps; (2) the use as polymerization catalysts of water-soluble percarbonates and perborates, hydrogen peroxide, benzoyl peroxide, acetyl peroxide, acetyl-benzoyl peroxide and lauryl peroxide; (3) the use as catalyst activators of reducing agents such as sulphites, bisulphites and hydrosulphites, of soluble salts of polyvalent metals, e.g. silver nitrate and copper sulphate, or of heavy metal complex salts such as alkali-metal ferricyanides and cobalticyanides; (4) the use for stabilizing the polymer of lead salts such as the carbonate, naphthenate, laurate and stearate, adipic dihydrazide, oxalic dihydrazide, and phenyl acetamidine; (5) the use as a criterion of the point at which polymerization should be stopped, of the solubility of the polymers in volatile solvents in addition to acetone, e.g. cyclohexanone, methylcyclohexanone, cyclopentanone, dioxane, methyleneethylene ether, tetra-methylene oxide and morpholine.