The mechanical behavior of compression-molded polyamide-6 (PA6) reinforced with 2 wt% of organo-nanoclay (montmorillonite intercalated with omega-amino dodecanoic: acid) has been studied and compared to that of PA6. The tensile strength and the Young's modulus of the PA6/clay were 15% higher than those of PA6. Differential scanning calorimetry, Fourier transform infrared spectroscopy, and X-ray diffraction showed that the crystalline structures of PA6 and PA6/clay differed considerably. A crystallinity of 25% with a dual structure composed of the gamma and alpha forms was obtained in PA6/clay, while a crystallinity of 31% with the a form as the dominant crystalline structure was obtained in PA6. To understand the role of the crystalline structure of PA6, the molding process was modified to obtain PA6 specimens with different levels of crystallinity and different crystalline forms. Quenching molten PA6 at a cooling rate sufficiently high to prevent crystallization gave a material that was predominantly amorphous (crystallinity of 7%) with traces of the mesomorphic beta or gamma* form. Annealing this material at 80degreesC considerably increased crystallinity to 21%, which was also of the mesomorphic beta or gamma* form. PA6 with a predominant crystalline gamma form could not be generated. Comparisons with PA6/clay in terms of crystallinity and mechanical behavior lead to the conclusion that the improvements in rigidity and strength observed when montmorillonite is added to PA6 are related to the reinforcing filler and not to a modification of the crystalline structure.