A fire-retardant polyester composition comprising a fiber-forming linear polyester having an intrinsic viscosity of at least 0.1 and a fire retardant containing phosphorus and bromine, wherein at least 70 mole % of the polyester consists of a recurring unit of the formula ##STR1## wherein n represents an integer of 2 to 6, and the fire retardant is a copolyphosphonate composed of 1 mole of a recurring unit of the formula ##STR2## wherein Y represents an isopropylidene or sulfonyl group, and R represents a phenyl or benzyl group,and 0.25 to 10 moles of a recurring unit of the formula ##STR3## wherein R represents a phenyl or benzyl group, the proportion of the fire retardant being 1 to 20 parts by weight per 100 parts by weight of the polyester.