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The influence of crystallization rate on the interesting polymorphism of random isotactic poly(propylene-co-1-heptene) copolymers was studied, with particular attention to the conditions for obtaining the mesomorphic phase. Owing to the important decrease of cooling rate involved as the 1-heptene content increases, the mesophase can be clearly observed by DSC for co-unit contents above about 5 mol%. Furthermore, only the mesophase (or the amorphous phase) was found for the copolymer with 13.9 mol% 1-heptene at common cooling rates of the calorimeter. Additionally, the copolymer with 21.4 mol% co-units was found to be completely amorphous under any thermal treatment analyzed. Variable-temperature diffraction experiments were carried out to ascertain unambiguously the nature of the phases involved. Finally, a remarkable transparency was attained (above 80% transmittance in the visible light region) at intermediate and high 1-heptene contents, using easily accessible cooling rates.