The structure and morphology of nylon 4 chain-folded lamellar crystals has been investigated using transmission electron microscopy (TEM), selected area electron diffraction, and X-ray diffraction. Solution-grown crystals are seen in the electron microscope in the form of sheaves with lathlike extremities. Electron diffraction patterns show the 0.44 (200) and 0.37 nm (020) reflections which are representative of the interchain distance within a hydrogen-bonded sheet and the intersheet distance, respectively, and are characteristic of the broader family of polyamides. The orientation of the electron diffraction patterns from individual lamellar crystals establishes that the polymer chains lie perpendicular to the lamellar surface and that the folding plane is parallel to the long axis of the crystal. Sedimented mats of the crystals were examined using wide-angle X-ray diffraction in a direction perpendicular to the lamellar normals. The X-ray diffraction patterns confirm that the nylon 4 chains are parallel to the lamellar normal. As a result of these findings and owing to the relatively high amide content of nylon 4, a unique mechanism for folding in nylons is proposed in which an amide group is incorporated in the fold, similar to that recently proposed for nylon 46. The fold exhibits similarity with the beta-bend in proteins rather than with the previous folding mechanism for nylons in which the alkane segments are in the fold. The crystallographic unit cell is monoclinic with a = 0.979 (+/-0.002) nm, b = 0.831 (+/0.002) nm, c (chain axis) = 1.225 (+/-0.002) nm, and gamma = 117-degrees and contains four chain segments or two sheet segments, similar to the unit cell of bulk nylon 6 and nylon 46. The effects of temperature on the lattice spacing show that the 200 and 020 reflections converge with increasing temperature but do not meet prior to the melting temperature, and two distinct reflections are observed up to the melt.