Intraspecific competition among blackbrush (Coleogyne ramosissima Torr.) seedlings was quantitatively investigated in an environmentally controlled glasshouse. Mortality of blackbrush seedlings was greatest in small-sized pots with high densities, while mortality was lowest in large-sized pots when growing alone. Significant interactions were detected between density and pot size (available growing space) for root:shoot ratio, root and shoot biomass, and shoot water potential among surviving seedlings. When examining density and pot size factors independently, significant differences were found in root:shoot ratio, shoot height, root and shoot biomass, leaf length, and shoot water potential of seedlings. Results of this common garden study revealed that mortality of blackbrush seedlings occurred in the absence of intraspecific competition, and that growth characteristics were significantly reduced with increasing neighbor density and decreasing available growing space among surviving seedlings.