The zebra mussel, Dreissena polymorpha (Pallas, 1771), is considered to be one of the world s worst invasive species with a large impact on local biodiversity and ecosystem services in Europe and North America. Recently, a large-scale displacement of the invasive zebra mussel by the similarly invasive quagga mussel, Dreissena rostriformis ... (Deshayes, 1838), is occurring in large parts of Western and Central Europe. While the exact reasons for the competitive advantage of the quagga mussel remain unknown, its potentially higher fitness might play a role. This replacement of one invasive species by a closely related invasive species offers a unique opportunity for unravelling patterns and processes of competition. To test whether the quagga mussel derives its competitive advantage from higher growth rates, a fully closed and controlled microcosm system was used to subject specimens of both species to different intensities of intraspecific and interspecific competition. The study revealed that both species reacted qualitatively similar to the different treatments. However, under all competition scenarios the quagga mussel showed substantially higher growth rates and larger growth ranges. Therefore, these characteristics might provide the quagga mussel with a higher flexibility in fluctuating environments and allow it to reach adult size earlier. This, in turn, can make the quagga mussel less prone to parasite pressure and other biological constraints during growth, and provides an advantage in the competition for space (hard substrates) and food.