Study of the pollination biology of endangered species can provide useful information for natural resource managers. We studied the floral biology of Xyris tennesseensis, a federally endangered species, at a site (Willett Springs) in north central Alabama. We described floral behavior and flowering phenology, supplemented pollen onto stigmas from self and outcross sources to determine its effect on seed production and germination ability, documented the floral visitor fauna, and collected data on behavior, abundance and pollen loads of floral visitor species during two years (1998–99). Flowering of the population lasted from July to September each year, but individual flowers completed blooming in about a 3-hr period beginning in late morning. On average, a single flower was visited by three insects for a total of five floral visits during its brief period of anthesis. Floral visitation was greatest when flowers first opened and declined steeply by the time flowers began to close. Halictid bees were the most abundant floral visitors, along with four species of pollen-consuming syrphid flies. Based upon quantity of pollen carried, location on the body, and visitation frequency, we concluded that the halictid bee Lasioglossum zephyrum was the single most important pollinator species on the study site. However, the pollen supplementation study showed that flowers produce seeds without visitation by insects, likely by self-pollination. We concluded that, unlike some endangered plant species, insect pollination is not required for seed production by X. tennesseensis.