Nonisothermal and isothermal crystallization experiments were performed on polypropylene mixed with carbon nanotubes produced by disproportionation of CO on Co-Mo catalysts. Functionalization of the nanotubes with octadecylamine made the tubes hydrophobic and allowed the tubes to be solubilized in an organic solvent. Mixing of the nanotubes with the polymer was accomplished by adding the nanotubes to a Decalin solution that contained dissolved polypropylene, followed by evaporation of the solvent. Dynamic mechanical analysis indicated very little difference in the small-strain mechanical properties between filled and unfilled polymers at the very low solid levels that were tested. By contrast, the crystallization behavior of the filled and unfilled polymer was quite different. Nanotubes promoted growth of the less-preferred beta form of crystalline polypropylene at the expense of the alpha form. In nonisothermal crystallization, the total amount of crystalline material in the sample was the same for the filled and unfilled materials. However, for isothermal crystallization experiments, the percent crystallinity in the filled materials was slightly higher. Most importantly, the rate of crystallization was substantially higher in the filled system. The results presented in this paper clearly show that carbon nanotubes nucleate crystallinity in polypropylene.