A structural characterization of samples of isotactic propylene butene copolymers (iPPBu), synthesized with a metallocene catalyst, is reported. The copolymers crystallize in the entire range of comonomer compositions, and a and gamma forms of isotactic polypropylene (iPP) crystallize in propene-rich copolymers, whereas form I of isotactic polybutene (iPB) crystallizes in butene-rich samples. Butene and propene comonomeric units are included in crystals of iPP and iPB, respectively, as indicated by the change of the unit cell parameters of copolymer crystals with changing comonomer composition. The trigonal form of iPP, recently found in propylene-hexene and propylene-pentene copolymers, does not crystallize in asprepared or melt-crystallized samples of iPPBu copolymers, but it has been obtained in oriented fibers of copolymers. In fact, in copolymers with butene concentrations lower than 10-15 mol %, crystals of a or gamma forms present in the melt-crystallized compression-molded samples transform by stretching at high deformations into the mesomorphic form of iPP, whereas in samples with butene concentration of around 50%, crystals of a form of iPP transform into the trigonal form of iPP by stretching. This is the first evidence of the crystallization of the trigonal form of iPP in iPPBu copolymers.