Nanocomposite films of isotactic polypropylene reinforced with cellulose whiskers highly dispersed with surfactant were prepared for the first time and compared with either bare or grafted aggregated whiskers. Films obtained by solvent casting from toluene were investigated by means of X-ray diffraction, differential scanning calorimetry, dynamic mechanical analysis and tensile testing. Evaluation of the crystallization behavior showed that the aggregated or surfactant-modified whiskers induced two crystalline forms (alpha and beta) in the nanocomposites and also acted as nucleating agents for isotactic polypropylene. The linear mechanical properties above the glass-rubber transition were found to be drastically enhanced for all three of the nanocomposites as compared to the neat polypropylene matrix, and these effects were attributed to a mechanical coupling between the polypropylene crystallites and filler/filler interactions. For the mechanical experiments at large deformations, the quality of the whisker dispersion was found to play a major role. The nanocomposites obtained with the surfactant-modified whiskers exhibited enhanced ultimate properties when compared to the neat matrix or to the composites containing the other filler types. (c) 2006 Elsevier Ltd. All rights reserved.