From 2003 to 2008, research on spatial and temporal dynamics of Corbicula fluminea was conducted at 10 sites along the Mondego Estuary (Portugal). A clear spatial and temporal variation in terms of density and biomass was observed, the highest values being found in the upstream oligohaline site, especially in warmer seasons. Seasonal density ranged from 4 to 11 500 individuals m 22 . Biomass ranged from 0.01 to 323 g ash-free dry weight m 22 . Comparing the results collected in the Mondego Estuary with other European and American invaded ecosystems, the density values are extremely high, but the biomass is lower than some of the reported values. This can be explained by the dominance of very small animals in the Mondego Estuary. Furthermore, the stepwise multiple regression model combining environmen-tal variables and biomass of C. fluminea predicted that the areas with higher values of temperature and phosphate, together with lower values of salinity, water flow and silt and clay, support higher biomass. In 2005, a large reduction in density was recorded and was related to the massive mortality of small individuals as a consequence of the severe drought that occurred in the Mondego Estuary.