A composition containing (1) a crystalline polypropylene resin and (2) one or more .beta.-nucleating agents of the formula (I), wherein R.sub.1, R.sub.2 and R.sub.3, independently of one another, are C.sub.1-C.sub.20alkyl, C.sub.2-Calkyl substituted by C.sub.1-C.sub.10alkylamino, di(C.sub.1-C.sub.10alkyl)amino, C.sub.1-C.sub.10alkyloxy or hydroxy; C.sub.3-C.sub.20alkenyl, C.sub.5-C.sub.12cycloalkyl, C.sub.5-C.sub.12Cycloalkyl substituted by 1, 2 or 3 C.sub.1-C.sub.10alkyl; cyclohexylmethyl; cyclohexylmethyl substituted by 1, 2 or 3 C.sub.1-C.sub.10alkyl; C.sub.5-C.sub.9cycloalkenyl, C.sub.5-C.sub.9cycloalkenyl substituted by 1, 2 or 3 C.sub.1-C.sub.10alkyl; phenyl substituted by 1, 2 or 3 radicals selected from the group consisting of C.sub.1-C.sub.10-oalkyl, C.sub.1-C.sub.10alkyloxy, hydroxy, halogen, trihalogenmethyl, trihalogenmethoxy, benzoyl, phenylamino, acylamino and phenylazo; C.sub.7-C.sub.9phenylalkyl, C.sub.7-C.sub.9phenylalkyl which is substituted on the phenyl by 1, 2 or 3 radicals selected from the group consisting of C.sub.1-C.sub.10alkyl, C.sub.1-C.sub.10alkoxy and hydroxy; naphthyl, naphthyl substituted by C.sub.1C.sub.10alkyl, adamantyl, or a 5 to 6 membered heterocyclic group; characterized in that the polypropylene resin has a content of .beta.-form crystals of at least 10% calculated by means of the following equation .beta.-form crystal content (%)=100.times.P.sub..beta.1,/(P.sub.a1+P.sub.a2+P.sub.a3+P.sub..beta.1) where P.sub.a1 to P.sub.a3 are respective peak heights (maxima) of the a-form and P.sub..beta.1 is a peak height (maximum) of the .beta.-form determined by wide angle X-ray scattering. ##STR00001##