Disclosed herein are poly(arylene thioether-ketone) fibers obtained by melt-spinning a thermoplastic material which comprises 100 parts by weight of a melt-stable poly(arylene thioether-ketone) (PTK) and optionally, up to 50 parts by weight of at least one of thermoplastic resins. The PTK has predominant recurring units of the formula ##STR1## wherein the --CO-- and --S-- are in the para position to each other, and has a melting point, Tm of 310.degree.-380.degree. C., a residual melt crystallization enthalpy, .DELTA.Hmc (420.degree. C./10 min) of at least 10 J/g, a melt crystallization temperature, Tmc (420.degree. C./10 min) of at least 210.degree. C., and a reduced viscosity of 0.3-2 dl/g as determined by viscosity measurement at 25.degree. C. and a polymer concentration of 0.5 g/dl in 98 percent by weight sulfuric acid. The PTK fibers of this invention have high heat resistance and strength.