Differential scanning calorimetry and wide angle X-ray scattering measurements of the cyclic trimer of 6-hexanelactam showed that, besides the basic crystalline form, A, obtained by slow crystallization from a water solution, the cyclic trimer can be prepared in two other crystalline forms, B and C, by heating to 174 and 216 degrees C, respectively. According to the analysis of vibrational and nuclear magnetic resonance (n.m.r.) spectra, the amide groups are present in the trans structure in all the crystalline forms of the cyclic trimer and, in forms B and C, the molecules of the cyclic trimer have the same conformational structure. Based on semiempirical and ab initio calculations of stable conformational structures and on structural interpretations of n.m.r. and vibrational spectra, a molecular structure with symmetry C-3 was proposed for the cyclic trimer in the forms B and C. Similarly to polyamides, the forms B and C of the cyclic trimer contain an amorphous phase, the molecular structure of which depends on the thermal history of the sample.