Three crystalline modifications of isotactic polybutene-1 have been obtained from dilute solution in a variety of habits. These three crystalline modifications are: (a) tetragonal, (b) ``untwinned'' hexagonal, and (c) orthorhombic. The structure of the orthorhombic polymorph has not previously been reported. The basic morphology of all forms is the thin lamellar structure associated with polymer single crystals containing folded chains. A crystal-crystal transformation (from tetragonal to ``twinned'' hexagonal) was observed to occur when single crystals were allowed to stand at room temperature. This transformation involved a conformational change (from an 113 helix to a 31 helix), as well as the crystallographic one, but it did not involve a morphological change. No change in either morphology or crystallography occurred at room temperature in the orthorhombic or the ``untwinned'' hexagonal polymorphs, although both could be transformed completely into the tetragonal form if dried crystals were heated at 90°C for one hour. If allowed to stand at room temperature, the resulting tetragonal crystals transformed to the ``twinned'' hexagonal structure. The isotactic polybutene system thus constitutes an excellent case in which crystallographic, morphological, and conformational changes may be studied.