Monodisperse 5-amide nylon 6 oligomers have been isothermally crystallized at 95 degreesC from NS,N-dimethylformamide. The crystals have been investigated using optical microscopy, X-ray diffraction, electron microsopy (imaging and diffraction), atomic force microscopy and computational modeling. The diffraction signals index on a two-molecule triclinic unit cell with the parameters: a = 0.967 +/- 0.002 nm, b = 0.540 +/-0.002 nm, c (molecular axis) = 4.56 +/- 0.02 nm, alpha = 49.3 +/- 0.5 degrees, beta = 90.0 degrees and gamma = 71.4 +/- 0.5 degrees. The calculated density is 1.15 g cm(-3). A structure refinement was undertaken based on the experimental electron and X-ray diffraction data. The results show that the molecules are in the all-trans conformation and hydrogen bond to antiparallel neighbors to form the usual nylon 6 hydrogen-bonded sheets. However in this structure, which has been called the lambda -structure, the sheets stack with progressive c-axis shear, and consequently, the molecular layer thickness is noticeably reduced. Successive molecular layers associate in crystallographic register with a periodicity of 3.31 nm to form three-dimensional prismatic crystals that can be seen using conventional optical microscopy. The lambda -structure is distinctly different from the traditional recuperative c-axis intersheet-sheared nylon 6 monoclinic alpha -structure. The 5-amide nylon 6 lambda -structure is a relative of the triclinic nylon 10 alpha*-structure and indeed, in some respects, bears an architectural resemblance to the nylon 6 6 triclinic alpha -structure.