Tentative evidence indicates that a discrete tussock may not be genetically uniform, especially if its vitality is reduced. Using molecular markers, we studied the genetic composition of eight vigorous and eight weak tussocks of Carex sempervirens (cover !75% and 35%, respectively) to elucidate the relationship between tussock vigor and genetic variability. Four C. sempervirens tussocks harbored more than one genotype. However, all four variable tussocks were weak, and the additional genotypes occurred only at the edge of the tussock base. The formation of a genetically variable tussock of C. sempervirens is best explained by a seedling or small tussock being incorporated by a large, expanding tussock that eventually surrounds it. The vitalityrelated pattern was most likely caused by the greater intraspecific competition both inside and around the vigorous tussocks. We conclude that the formation of a tussock may involve more than one seedling and that tussocks, therefore, may not a priori be treated as genetic individuals.