Oriented specimens of isotactic cis-1,4-poly(3-methyl-1,3-pentadiene) exhibit two crystalline modifications characterized, respectively, by the expected two-fold helical conformation with a fiber repeat of 8,02 Angstrom and by a three-fold helix with c = 9,97 Angstrom. The latter highly contracted structure is prevalent in fibers and is the only one observed in unoriented bulk specimens. Consistent with these findings conformational calculations indicate that the three-fold helix is substantially more stable than the two-fold one.