Wet-spun fibers of a diacetylene-containing aliphatic polyamide (PADA 6,22) were exposed to controlled dosages of electron-beam radiation to selectively crosslink the crystalline regions via a topochemical diacetylene to polydiacetylene conversion. For aligned polymer chains in an oriented fiber, the polydiacetylene crosslinks are created perpendicular to the fiber direction; interference microscopy revealed that the refractive index increased in this direction. Dynamic mechanical spectroscopy and torsional modulus measurements showed that the noncrystalline phase remains essentially unaffected by electron-beam irradiation up to 60 Mrad. Thermal stress analysis demonstrated that higher shrinkage stresses are retained by the irradiated fibers at temperatures approaching the effective thermomechanical melting temperature of the nonirradiated fiber (200 degrees C); thermal deformation analysis also revealed that much lower extensions are exhibited by the crystalline-phase crosslinked fibers at these temperatures. The results suggest that crystalline-phase crosslinking of functionally modified semicrystalline polymers constitutes a mechanism for enhancing structural integrity at elevated temperatures without reducing amorphous-phase flexibility.