The influence of pollen competitive environment on pollen performance (pollen germination, stigmatic penetration, and pollen tube growth rate), the maturation or abortion of initiated fruit, seed size, and seedling vigor was explored by manipulating the size and diversity of stigmatic pollen loads on Mirabilis jalapa. All aspects of pollen performance significantly increased with the number of pollen grains on a stigma or pollen tubes in a style, but was not influenced by the diversity of pollen donors. Plants tended to mature single-ovulate fruits that came from flowers where pollen load size and diversity were greatest and aborted those where these were lowest. No plants from seeds resulting from pollinations with a single pollen grain survived, but other fitness measures were mostly determined by maternal plant. The data suggest that pollen performance is influenced by pollen competitive environment, and both the genetic diversity of the pollen load and number of competing pollen tubes are important determinants of seed/fruit abortion.