Pollination biology and factors affecting reproductive success of the federally endangered Echinacea laevigata (coneflower) were studied in one large and five small populations. Insect visitor exclusion from flowering heads and pollination treatments showed that seed production was 10x higher in open-pollinated treatments compared to bagged flowers that were not pollinated and 20x higher than flower heads receiving self pollen suggesting that cross-pollination was responsible for most seed production. Peak flowering occurred the 2nd–4th wk of Jun. 2004 and the large population contributed 50–68% of the flowering individuals in its plant community. During this period, Bombus (Apidae, bumblebees), Hesperiidae (skippers), and Megachile (Megachilidae, leaf-cutter bees) accounted for 73%, 12%, and 11% of the observed visits to coneflower, respectively. Species richness of coneflower flower visitors and number of flower visits was lower in each small population than in the large population. However, comparison of seed production between heads receiving supplemental pollen and open-pollination heads indicated that seed production was not pollen limited. In addition, over 93% of pollen grains stained normally in cotton blue-lactophenol in all six populations suggesting that pollen viability was high in small and large populations. However, seed production in the large population exceeded that in each of three small populations during 2005, and seedling size was larger in the large population compared to seedlings from one of the small populations. Comparison of soil cation concentrations among populations suggested that cations did not limit seed production. We suggest that genetic factors, such as inbreeding or low S allele variation, may limit seed production in the small populations.