An isotactic polypropene (i-PP) prepared with a metallocene catalyst system and two random copolymers of propene and ethene with 2.8 and 5.7 wt % ethene, respectively (PP-co-E-2.8, PP-co-E-5.7), were isothermally crystallized during simultaneous small- and wide-angle X-ray scattering (SAXS, WAXS) in synchrotron measurements. Identical measurements were carried out after adding 0.3 wt % N,N'-dicyclohexyl-2,6-naphthalene dicarboxamide, a beta nucleating agent (beta -NA). All samples form the gamma -modification to a certain extent. WAXS data show that the alpha -modification is predominant in i-PP, PP-co-E-2.8, and PP-co-E-5.7 isothermally crystallized without beta -NA at temperatures below 125 degreesC. Adding beta -NA leads to significant amounts of the beta -modification at all crystallization temperatures in all three samples. Thus, the three modifications can be tailored by the crystallization regime of the nucleated samples. The simultaneous occurrence of the three modifications has also implications on the SAXS data. The formation of the gamma -modification in addition to the alpha -modification does not lead to a one-dimensional correlation function K(z) with two distinct long periods or lamella thicknesses. In contrast, the formation of the beta -modification results in a long period which can be distinguished from that of the other modifications after generating K(z).