The effect of intraspecific competition and nutrient supply on the endangered northeastern bulrush, Scirpus ancistrochaetus, was examined in a full-factorial greenhouse experiment. Seedlings were exposed to 4 densities (1, 5, 10 or 25 plants/pot) and 3 nutrient concentrations (3, 25 or 50% of full strength) for 9 wk. Height, mass and survival generally decreased with increasing density. Root to shoot ratio (R : S) increased with density, indicating that competition primarily involved belowground resources. Nutrient concentration significantly affected average plant height, total mass and R : S but did not affect survival. Plant height and total mass generally increased with increasing nutrient concentration, whereas R : S decreased. In general, the negative effect of intraspecific competition on plant height and total mass was greatest at the highest nutrient concentration, confirming that the intensity of intraspecific competition increases with resource supply in this species. Neither density nor nutrient concentration affected the population distribution of plant sizes in densities over 1 plant/pot, suggesting that size hierarchies, and therefore asymmetric competition, are not important during seedling growth in this species. Information gained from this study can be used towards understanding seedling establishment in the field and, in turn, towards effective management and reintroduction of this endangered species.