The structure and morphology of lamellar crystals of nylon 12,5 have been investigated by transmission electron microscopy, electron diffraction and X-ray diffraction. The results obtained indicate that this nylon has a structure different from the conventional alpha, beta and gamma forms. Its features are similar to those previously reported for nylon 6,5, which has been shown to have its molecules packed in a network of hydrogen bonds with two spatial orientations (Navarro et al., Macromolecules, 28, 8742). Thus nylon 12,5 should be included in a new family of aliphatic polyamides. The structure derives from a particular conformation of the glutaric units and appears to be independent of the number of methylene groups present in the diamine residue. As a function of the method used in older to prepare the sample, a pseudohexagonal form can also be observed. It might be related to the gamma form.