The crystal structure of the trigonal form of isotactic polypropylene in propylene-pentene random copolymers is presented. These copolymers crystallize in the a form of iPP for concentrations of pentene lower than 7-8 mol % and in the trigonal form for higher pentene concentration. The trigonal form does not crystallize by cooling from the melt but crystallizes from the amorphous by cold-crystallization or for samples with high pentene concentration by aging the amorphous samples at room temperature. The pentene units are included in the crystals of trigonal form and, at low concentration, also in the crystals of alpha form, producing increase of the unit cell dimension. The change of crystallization habit from monoclinic into trigonal occurring for concentrations higher than 10 mol % allows incorporation of higher amounts of pentene units in the crystals of the trigonal form than in the alpha form. The crystal structure of the trigonal form has been studied by analysis of the X-ray fiber and powder diffraction patterns and electron diffraction of single crystals of propylene-pentene copolymers having concentrations of pentene units in the range 20-54 mol %. Chains of propylene-pentene copolymers in the 3-fold helical chain conformation are packed in a trigonal unit cell according to the space group R3c or R (3) over barc. The dimension of the unit cell axes depends on the amounts of pentene units included in the crystal lattice, and when pentene concentration approaches 50 mol %, the value of a = b axes becomes practically equal to that of the unit cell of the stable form I of isotactic polybutene.