The crystallization and thermal properties of copolymers of isotactic poly(1-butene) (iPB) with ethylene (iPBEt) synthesized with a Ziegler-Natta catalyst and ethylene (E) content between 7 and 45 mol % are presented. The crystallization properties provide evidence of a multiblock molecular structure of these copolymers. The samples have been separated in two or three fractions by standard fractionation in boiling solvents. Samples with ethylene concentration lower than 16 mol % surprisingly crystallize in form II of iPB, and crystallization of polyethylene also occurs for the maximum ethylene content of 45 mol %. Even more surprising is the result that the melting temperature of crystals of any polymorphic form of iPB that crystallize in these samples do not depend on the ethylene concentration. This suggests that long crystallizable butene sequences are present even at high ethylene concentration. In all fractions, the chains are composed of multiblocks where the comonomers are segregated, with blocks characterized by long crystallizable butene sequences and blocks with shorter butene sequences. In the different fractions, the average length of butene sequences is nearly the same. The different fractions are featured by different amounts of the constituting blocks.