Pollen dimorphism has been intensively studied in many species of Angiosperms but not in the Onagraceae. Some species of Oenothera have been reported to present differences in the sizes and starch contents of their pollen grains but most of the research done in the ultrastructure of pollen an microsporogenesis has been performed in species that do not present any pollen polymorphism. In both O. picensis and O. villaricae, each haploid set of chromosomes is united into a ring called a Renner complex. These Renner complexes are each transmitted intact from one generation to the next and, since Oenothera picensis and Oenothera villaricae form fully fertile hybrids when crossed in either direction, it is possible to construct several different combinations of Renner complexes and cytoplasmic backgrounds. In this work, I show the existence of pollen polymorphism in Oenothera picensis, Oenothera villaricae and their hybrids, report on the origin of the polymorphism during microsporogenesis and provide evidence that the different pollen sizes are representing two different genome. I present the hypothesis and evidence that this polymorphism is a morphological and physiological expression of the different genetic characteristics of the pollen grains. Finally, I will indicate how these differences may be related to the phenomenon of non-random fertilization associated with these species of Oenothera.