Two crystalline structures for nylon 5,6 have been studied by transmission electron microscopy, electron diffraction, and X-ray diffraction. A gamma form or an alpha-like phase can be obtained as a function of the method used to prepare the sample. Structural modeling and energy calculations have been carried out for the second form. Results point to a structure with two hydrogen bond directions as recently postulated for even-odd nylons and odd polyoxamides. It appears therefore that in even-odd and odd-even nylons an alpha-like structure may be frequently observed, but with two hydrogen bond directions. Our results strongly favor for nylon 5,6 a model in which the methylenes are in an all-trans conformation, while the torsional angles for the CH2-NH and CH2-CO bonds of one residue respectively are phi = +152 degrees and psi = +/-137 degrees. Temperature-induced structural changes have also been studied for nylon 5,6 fibers. Pseudohexagonal packing is obtained near 220 degrees C in a process which takes place without changes in the hydrogen bond system.