Carex bigelowii an arctic-alpine sedge that shows an extensive clonal growth, and in many years also has a high degree of flowering. To see which factors are most important in determining the sexual reproductive success, the effects of self- and cross-pollination, pollen donor distance, amount of pollen, temperature, and time of flowering were studied. The study was conducted at Latnjajaure Field Station, northern Sweden and Thingvellir National Park, Iceland during three field seasons. Experimentally increased air temperature raised the fruit set and the fruit weight, in two of the three seasons. Pollen donor distance did not influence either fruit set or fruit weight, and there was no difference in fruit weight between selfing and cross-pollination. An increased amount of pollen raised the fruit set. but did not affect fruit weight. The time of flowering was also important; there was a negative relationship between flowering time and fruit set and fruit weight, i.e. late flowering ramets had a lower fruit set and fruit weight. The results suggest that Carex bigelowii is an outcrossed species where the reproductive success is mainly determined by the weather, and that there are differences between the two study sites.