Tiller dynamics and net aerial primary productivity (NAPP) were studied in permanent sample plots of Spartina densiflora at Otamendi Natural Reserve (Buenos Aires Province, Argentina) from June 1996 to December 1997. Emergence, growth, senescence, survival, biomass dynamics, and tiller productivity were analyzed. These population attributes varied seasonally during the first study year, whereas variations were not detected in the subsequent year. Tiller density increased slightly during the first twelve months, attaining 2445 ± 869 tillers·m⁻², then remaining constant until the end of the study period. Senescence rate was lower than emergence rate on most sampling occasions. Cohorts originated in different seasons showed distinct survival curves. The life span of tillers was at least 18 months. Green biomass reached maximum values (771 ± 410 g·m⁻²) during the first year but decreased during the last six months of observations. Annual NAPP was 1450 ± 566 g·m⁻²·y⁻¹. Average productivity rate was higher in the first year than in the second year. The decrease in the population density of tillers of S. densiflora and the increase in its senescence rate were more likely related to grazing and flooding than to population dynamics.