PURPOSE: To provide a filament of a fiber-forming polyester copolymer composed of a new polyester copolymer containing respectively specific plural structural units and exhibiting high strength and modulus by heat-treatment.

CONSTITUTION: The objective fiber is produced by the conventional spinning of a fiber-forming polyester copolymer containing 50-70 mol% of a unit of formula I (X is H, methyl or Cl), 10-25 mol% of a unit of formula II, III or IV (R is X), 10-25 mol% of a unit of formula V and further 0-10 mol% of a unit of formula VI selected from 1,3- and 1,4-dihydroxybenzenes and 0-5 mol% of a unit of formula VII. The above copolymer can be produced e.g. by the polymerization of 4-acetoxybenzoic acid, 3,4'-diacetoxybenzophenone, terephthalic acid and resorcinol diacetate.