Oenothera villaricae Dietrich and Oe. picensis Dietrich, complete translocation heterozygotes, are fully interfertile, giving rise to six discrete classes of true­ breeding hybrids from a reciprocal cross. Associated with each parent and hybrid is a characteristic ab01tive non­ staining pollen fraction easily distinguished from fully developed pollen under the light microscope. Pollen abortion has been associated with translocation rings in other angiosperm species, and may characterize such sys­ tems. The abortive pollen fraction is significantly differ­ ent between reciprocal Oenothera hybrids, however (P<0.001), indicative of partial cytoplasmic control. Pol­ len abortion is most severe in the F 1 hybrid generation, and ameliorates with successive generations of hybrid self-fertilization. Three-way analysis of variance shows significant effects on pollen stainability (a measure of the non-abortive fraction) for nucleus, cytoplasm and selfed hybrid generation, individually or in combinations. This result suggests a combined nucleocytoplasmic basis for the pollen abortion. Correlated with the observation of in­ creased pollen abortion in Oenothera hybrids are meiotic findings of broken chromosome rings (chains, univa­ lents), asymmetric anaphase chromosome distributions and trinucleate tetrads. To test the hypothesis that such anomalous meiotic events play a role in the mechanism of pollen abortion, meiotic disjunction frequency was de­ termined for each parent, F1 and F9 selfed hybrid acces­ sions. Three-way analysis of variance shows levels of sig­ nificance comparable to those noted for pollen stainabili­ ty (P