The three crystalline phases of isotactic poly(1-butene), which differ by their helix conformation and unit cell parameters, are obtained by bulk crystallization via epitaxy on appropriate organic substrates, in particular aromatic acids or salts. In this paper, the epitaxial relationships between forms III and II are established, based on composite electron diffraction patterns where possible. Form III epitaxy rests mainly on the matching of the chain axis repeat distance; form II is an original illustration of an epitaxy of irrational helices in which the interturn distance plays the main role.