Abstract: A novel copolyamide fiber having an excellent initial modulus of elasticity and mechanical strength, is made from a copolyamide consisting essentially of unit A of the formula (A): ##STR1## unit (B) of the formula (B): ##STR2## and unit (C) of the formula (C): ##STR3## wherein CH.sub.3 groups in the formula (A) are located in the ortho-position to NH groups, the sum of the amount of units (A) and (B) is substantially equimolar to the amount of unit (C) and the molar ratio of unit (A) to unit (B) is from 10/90 to 50/50.

Abstract: Fibers of a copolyamide derived from a diamine component including 10 to 50 mole % of benzidine sulfone or 2,7-diaminophenanthridone and 90 to 50 mole % of p-phenylene diamine and a dicarboxylic acid component derived from a reactive derivative of terephthalic acid are disclosed. The fibers have an initial modulus of elasticity of at least 400 g/d and a tensile strength of at least 16 g/d.The fibers have in combination various desirable properties, such as excellent initial modulus of elasticity, high tensile strength, high knot strength, high heat resistance and good adhesiveness to various plastics, rubbers and adhesives, and they are valuably used in various industrial fields as fibrous reinforcers and for production of fiber-reinforced plastics and rubber products.