Crystals of the monodisperse 5-amide nylon 6 oligomer (4.6 nm long), grown from solution, have been investigated using X-ray diffraction and electron microscopy. If the crystals are prepared by adding the miscible non-solvent 1,4 dioxane to a solution in trifluoroethanol the usual nylon 6 monoclinic a-structure occurs, i.e. a structure where the hydrogen-bonded sheets shear in an alternating fashion with respect to the chain (c-axis) direction. If, however, crystals are grown isothermally from NN-dimethylformamide at 70-80 degreesC a progressive c-axis intersheet-sheared triclinic structure is found. These crystals are three-dimensional in character, owing to the crystallographic stacking of successive molecular layers (3.4 nm thick due to chain tilt), and the diffraction signals index on a triclinic unit cell with parameters: a = 0.980 +/-0.002 nm, b = 0.545 +/- 0.002 nm, c = 4.60 +/- 0.02 nm, alpha = 50.4 +/- 0.5 degrees, beta = 90 degrees and gamma = 71 +/- 0.5 degrees. A model is proposed for this triclinic structure and the calculated diffraction pattern is compared with experimental data. In both monoclinic and triclinic polymorphs the 5-amide molecules exist in an all-trans non-folded conformation. Crystals of the 9-amide nylon 6 oligomer have also been prepared using 1,4 dioxane/trifluoroethanol mixtures. In this case, these longer 9-amide molecules (8.06 nm) are once-folded but again crystallize in hydrogen-bonded sheets that stack in the usual nylon 6 monoclinic a-structure. When these crystals are annealed at 140 degreesC, a solid state transformation occurs into a triclinic structure with progressive c-axis intersheet shear,