We studied the floral biology, floral visitors, levels of florivory, and mating system of Opuntia macrocentra in a population of ca. 300 individuals in order to describe what factors affect flower/fruit ratios. Blooming for the species occurred once a year during spring. Flowers were hermaphroditic, produced nectar, and remained open 6 to 9 h during a single day. Anther dehiscence starts at flower aperture and stigma receptivity starts approximately 1 h later. The most important floral visitors were solitary bees from the Anthophoridae family (genus Diadasia). Open- pollinated control and cross pollination treatments had the highest fruit set (96.8 ± 3.2% and 83.9 ± 6.7%, respectively), but fruit set for forced self-pollination treatment (77.4 ± 7.6%) did not differ from the cross-pollination treatment. Seed production was also highest in the open-pollinated treatment; the average number of seeds per fruit in the open-pollinated treatment was 40% higher than the cross-pollinated treatment and 64% higher than the self-pollinated treatment. The flowers were self-compatible and did not require a visitor to set fruit. Flower/fruit ratio was slightly above one over all pollination treatments (fruit ratios between 1.0–1.3), suggesting that almost all flowers turned into fruits. Outcrossing rates suggest a mixed mating system, but inbreeding depression was found for both fruit and seed set. Developing fruits were consumed by the caterpillar (Lepidoptera: Olycella subumbrella) and decreased fruit set from 20% to 100%. Florivory and inbreeding depression were the major factors that decrease fruit set for this species.