The false mussel, Mytilopsis leucophaeata originating from the Atlantic coast and Gulf of Mexico in North America invaded brackish water systems in Europe. Seasonal sessile population structure is determined by spat fall, growth, and mortality. Juvenile and adult M. leucophaeata were kept in cages in a brackish water canal to study their mortality and to estimate annual and seasonal shell growth rate in relation to shell size and water temperature. Mortality increased with increasing temperature with the strongest effect on the 4–6 mm and 8–10 mm size classes. The average mortality of all size classes of M. leucophaeata in cage experiments was lowest in winter, increased from April to July, and peaked in July and August. Between a water temperature of 18 and 23°C, a strong growth was observed for mussels of size classes 2–14 mm, whereas mussels of size class 14–16 mm showed only a slight growth (<20 µm day^-1) and size classes 16–18 mm and 18–22 mm showed no growth at all. When water temperature was between 9 and 18°C, there was no or slight growth, and at lower than 9°C, hardly any growth. Especially for the small mussels, the influence of the water temperature on shell growth is strong. Polynomial regression showed a mean summer growth rate of 94 µm day^-1 for the 2–4 mm class, 37 µm day^-1 for the 10–12 mm class, and no growth for the 18–22mmclass. Analysis of all available (literature) data yielded mean summer and annual growth rates of M. leucophaeata collected in brackish western European harbors and canals ranging from 30 to 133 µm day^-1 and 8 to 49 µm day^-1, respectively. Increase of water temperature by climate change or thermal discharges will increase growth rates and mortality of this mussel.