Microsporogenesis, embryo sac development Including fertilization, and fruit set involving cross pollinations were studied in the highly self-fertile red raspberry (Rubus idaeus L.) cultivar Mailing Jewel and the partially male sterile United States Department of Agriculture -Oregon State University selection Ore-US 1314 from the United States Department of Agriculture-Oregon State University red raspberry breeding program. Pollen was stained with aceto-carmine to determine pollen abnormality. Anthers of different filament lengths differed in the percent of normal pollen produced in the Ore-US 1314. Percent normal pollen produced by this selection is also much less than that, produced by Mailing Jewel. Longitudinal and cross sections of the anthers of both clones were examined and no direct involvement of the tapetum was observed to be contributing to the production of abnormal microspores in either clone. Anther squashes showed that only 41.8% of the cells of Ore-US 1314 underwent normal meiosis as compared to 93% in Mailing Jewel. Lagging chromosomes at meiosis I were also observed in Ore-US 1314. Cytogenetic factors are thought to be involved in causing increased pollen abortion and thus male sterility in Ore-US 1314. Examinations of embryo sac development under open pollination showed that while Ore-US 1314 did riot have a retarded embryo sac development, it had a high percentage of degenerate embryo sacs. Degeneration is believed to be a result of pollen of poor quality, incompatible pollen, the collapse of the egg cell due to failure of pollen to fertilize it at the critical time, or the involvement of all three. The pollination experiments involved self, open, and cross-pollinations of the two clones. The results obtained from the druplet set suggest that Ore-US 1314 could be both partial and cross-incompatible. Druplet set in Ore-US 1314 when open pollinated increased above that when selfed suggesting a positive reaction to pollen mixtures. Mailing Jewel, although not self-incompatible, was found to be partially cross-incompatible with Ore-US 1314.