The competition between alpha, beta and gamma polymorphs has been studied in a P-nucleated metallocenic isotactic polypropylene, iPP, as a function of the cooling rate and of the isothermal crystallization temperature, by performing X-ray diffraction and DSC experiments. It was found that the addition of a 1% by weight of a typical P-nucleating agent is not enough to develop any appreciable amount of beta modification, at least under the crystallization conditions used, which cover a wide range of cooling rates. In comparison, the same amount of nucleating agent added to a Ziegler-Natta iPP leads to almost 100% of beta form at low cooling rates. It seems that such amount of beta nucleating agent is not enough to counterbalance the well-known gamma nucleation ability of the relatively high content of defects (stereo-and regioerrors) which are present in the studied metallocenic iPP, and only different proportions of gamma and a modifications are obtained in this sample, the relative amount of them depending on the cooling rate. On the contrary, if a 5% nucleating agent is added, the beta modification is also obtained, in addition to the gamma and alpha polymorphs. However, now the amount of beta crystals as a function of the cooling rate follows a trend opposite to that for the Ziegler-Natta iPP: the higher are the cooling rates (or the lower are the isothermal crystallization temperatures) the larger proportions of beta modification are obtained. It is deduced, therefore, that the nucleation ability of the chain errors which leads to the development of the gamma form predominates over that one of the nucleating agent. The enthalpies for the 100% crystalline modifications, estimated from the enthalpies of melting and from the X-ray determined proportions of the different polymorphs, are rather similar: 162, 159, and 158 J/g for the alpha, beta and gamma phases, respectively. These values are inside the experimental error.