As seed and fruit size can influence the success of later life history stages, it is important to understand their sources of variation. In this study, we examined the effects of pollen load and donor diversity on variation in seed and fruit mass of outcrossing senita cacti {Pachycereus schottii) in the Sonoran Desert. By massing all individual seeds per fruit from pollen supplementation and donor diversity experiments, we were able to examine their effects on seed and fruit mass, compared with intra-and interplant variation. Seed and fruit mass showed up to 13-and 15-fold variations, respectively. Pollen load did statistically increase seed and fruit mass, but it explained <6% of their variation. Pollen donor diversity did not affect seed or fruit mass. Variation in seed and fruit mass was explained by interplant variation (19%), intraplant variation among fruits (30%), and intrafruit variation (50%). These results indicate that intra-and interplant sources, excluding pollen load and donor diversity but possibly including environmental, architectural, and maternal effects, contribute to the substantial variation observed in seed and fruit mass of senita cacti. Such variation may prove useful for plant recruitment under the highly variable water-and nutrient-stressed conditions of desert environments.