Northern Wild Senna (Senna hebecarpa) is a native perennial wildflower that was once common in New England, but is now restricted to only seven extant populations in the region. The species thrives in moist alluvial soils along riverbanks and roadside ditches, and its decline has been attributed to forest succession, human development, and changes in hydrology. Given its increasing rarity, individual plants are becoming more isolated, which creates the potential for reduced pollination if the plant is self-incompatible or the potential for reduced fitness from inbreeding depression if it is self-compatible. Results from pollination experiments confirmed that the species is self-compatible, but requires a pollinator to effect pollination. Since the species is self-compatible and therefore capable of inbreeding, we examined the impact of selfing on fitness at both the level of parental fecundity and progeny success. Comparisons between fully selfed and crossed plants revealed that inbred plants had on average slightly lower fecundity, and that their progeny had on average slightly lower viability and fecundity. To better understand the impact of inbreeding on fitness we extended our analysis of individual measures of fitness to include cumulative effects. These results showed that when the small, marginally significant differences for individual fitness parameters were taken together over the course of successive life history stages the net result was that selfed plants produced significantly fewer viable progeny. This finding shows that S. hebecarpa is sensitive to a loss of fitness due to inbreeding depression, which has implications for its conservation.