We analyzed the relationships between population size and reproductive characteristics in the perennial prairie forb Phlox pilosa , an obligate outcrossing butterfly-pollinated species. We examined 27 populations ranging in size from 9 to over 75,000 flowering ramets in two regions of the state of Iowa (eastcentral and northwest) in 1993 and. We collected flowers from each population and scored them for pollen arrival to stigmas and number of pollen tubes. We collected fruiting ramets from each population at the end of the Phlox growing season and scored them for height, biomass, and reproductive variables, including the number of flowers initiated and opened and the number of capsules initiated and matured. In both years, population size was significantly correlated with the number of capsules matured per ramet. Differences between populations in capsule production were set primarily at the pollination stage. In 1993, pollen arrival to stigmas was significantly lower than in 1994 and was correlated with population size in eastcentral Iowa populations, indicating that lower reproduction in small populations that year was at least partially due to inadequate amounts of pollen being moved. In 1993, weather conditions likely depressed pollinator activity, but absolute capsule formation was high because of high flower production per ramet and high population densities. In 1994, when pollen arrival to stigmas was relatively high and unrelated to population size, outcross pollen movement was greater in larger populations. Increased efficacy of outcross pollen movement in 1994 may have resulted from lower flower production and less dense populations forcing greater pollinator movement between ramets or from variation between years in fine-scale spatial genetic substructuring of populations. Our results indicate that the viability of Phlox pilosa can be best ensured by protecting and creating populations of at least 1000-2000 flowering ramets.Tamaño Poblacional y Reproducción de Phlox pilosa Resumen: Analizamos las relaciones entre el tamaño poblacional y las características reproductivas en la hierba perenne de pradera, Phlox pilosa , una especie obligada a cruza externa, polinizada por especies de mariposas. Examinamos 27 poblaciones que variaron en tamaño entre 9 y 75,000 ramilletes en dos regiones del estado de Iowa (este-centro y noroeste) en 1993 y 1994. Colectamos flores de cada población y establecimos puntajes para el arrivo de polen a las estigmas y número de tubos de polen. Colectamos ramilletes de frutas para cada población al final de la estación de crecimiento de Phlox y establecimos puntajes para altura, biomasa y variables reproductivas incluyendo el número de flores iniciadas y abiertas y el número de cápsulas iniciadas y maduras. En ambos años, el tamaño poblacional estuvo significtivamente correlacionado con el número de cápsulas maduras por ramillete. Las diferencias entre poblaciones en producción de cápsulas se establecieron primariamente en el estado de polinización. En 1993 el arrivo de polen a l...