Trager, M. D. (Dept. of Wildlife Ecology and Conservation, University of Florida, Gainesville, FL 32601), E. S. Menges (Archbold Biological Station, PO Box 2057, Lake Placid, FL 33862), P. F. Quintana-Ascencio (Dept. of Biology, University of Central Florida, Orlando, FL 32816), and C. W. Weekley (Archbold Biological Station, PO Box 2057, Lake Placid, FL 33862). Outcrossing effects on the reproductive performance of Hypericum cumulicola, an endangered Florida scrub endemic. J. Torrey Bot. Soc. 132: 204–213. 2005.—We investigated the effects of self-fertilizing and within-population outcrossing in two populations of Hypericum cumulicola, a perennial herb endemic to xeric scrub habitats on central Florida's Lake Wales Ridge. To examine the relationship between pollen source and reproductive performance, we compared fruit set, seed set, percent germination and net fertility (mean number of seedlings produced per flower within a pollination treatment) of hand-pollinated selfed and outcrossed flowers. There were no significant effects of categorical pollen source (self, near outcross, far outcross) on fruit set, and logistic regression showed no effect of interplant distance. Overall and at one of the two populations, far outcrosses had higher seed set than selfs and we found a weak but significant positive correlation between outcrossing distance and seed set. Percent seed germination was unaffected by pollen source. Net fertility was significantly correlated with outcrossing distance overall and at one of the two sites. These results show that pollen source may influence reproductive performance in Hypericum cumulicola, but the effects are fairly weak and inconsistent among populations, suggesting that outcrossing distance is not an important factor affecting small-scale population dynamics of this species.