The structure and morphology of lamellar crystals of a sequential copolymer of poly(pirrolidinone) with poly(epsilon-caprolactam) (nylon 4/6 copolymer) has been studied by transmission electron microscopy, electron diffraction and X-ray diffraction. The results obtained indicate that at room temperature single crystals of this copolymer have an alpha-form structure which is also the most stable one for both related homopolymers. The crystallographic unit cell is monoclinic with a = 9.60 Angstrom, b (chain axis) = 14.80 Angstrom, c = 8.06 Angstrom and beta = 67 degrees. The effects of temperature on the lattice spacings show that the characteristic 200 and 002 equatorial spacings converge with increasing temperature but do not meet prior to the melting temperature. In conclusion, the copolymer and the related homopolymers behave similarly with respect to their room temperature most stable crystal structure and behavior on heating. The second characteristic crystalline phase (gamma-form) reported for even nylons could only be obtained by treating the polymer sample with iodine-potassium iodide solutions.