The crystal polymorphism of isotactic poly(5-methyl-hexene-1) (P5MHex1) is reexamined on the basis of new data: electron diffraction of solution- and thin film-grown single crystals, fiber patterns from stroked films. Preliminary crystal structures are determined by conformational analysis and packing energy minimization. A crystal structure initially described as orthorhombic (Corradini, P.; et al. Eur. Polym. J. 1970, 6, 1201) is actually a frustrated, chiral phase with a trigonal unit-cell (a = b = 17.62 angstrom, c = 6.33 angstrom, gamma = 120 degrees, space group P3(1) or P3(2)). Analogies between the structures of isotactic P5MHex1 and poly(5-methyl-heptene-1) reported by Corradini et al. suggest that the latter polymer has also a frustrated structure. The frustrated character is lost upon heating. The high temperature form is best described as a one chain, trigonal unit-cell with probably significant side chain mobility (a=b = 10.17 angstrom, c = 6.33 angstrom, gamma = 120 degrees, space group P3(1) or P3(2)). Interconversion on heating and cooling between the frustrated and the one-chain unit-cells is probable. An original crystal modification formed during synthesis of P5MHex1 but not previously analyzed is obtained as single crystals in dilute solution. The monoclinic cell (a = 20.48 angstrom, b = 16.94 angstrom, c = 6.33 angstrom, but alpha = beta = gamma = 90 degrees) houses four 3-fold helices (two pairs of enantiomorphous chains). This metastable form converts by melting-recrystallization to the chiral, frustrated phase. On standing at room temperature, a structural rearrangement takes place in the frustrated form, and a packing of layers similar to the monoclinic phase develops. The rich polymorphism of P5MHex1 illustrates some of the many packing modes available to 3-fold helices of polymers with long, flexible side chains.