Colloidal boehmite particles have been included into a polyamide-6 matrix by in situ polymerization. The particles have been used without any surface modification. Characterization of the nanocomposites has been carried out using transmission electron microscopy (TEM), dynamical mechanical analysis (DMA) and differential scanning calorimetry (DSC). TEM images indicate that the particles have been homogeneously dispersed in the polymer. DSC results show that the presence of boehmite affects the form of crystallization of polyamide-6, in which case formation of gamma-structure is favored over the alpha'-structure and an additional alpha'-phase is formed. Some mechanical reinforcement of the matrix has been accomplished as indicated by DMA results. The modulus level off at high boehmite concentrations can be explained by the reduction of the crystallinity, which cancels the effect of the filter. (C) 2004 Published by Elsevier Ltd.