Examining congeners can help identify critical differences between species that affect invasion and spread. We examined Dreissenapolymorpha (zebra mussel) and Dreissena rostriformis bugensis (quagga mussel), which are important invaders in freshwater and share general ecological characteristics, to determine whether they had similar rates of invasion at different spatial scales (global, regional, local, and individual water bodies) from the time of first introduction to the present. We also contrasted differences in ecological and population characteristics that could influence speed of spread. Although D. polymorpha and D. r. bugensis are relatives and share a common native habitat, morphology, lifestyle, life history, and dispersal potential, D. polymorpha was found to be a better invader than D. r. bugensis at most spatial scales throughout their invasion history. Spread at the regional scale in North America was the same for both species, but the initial rate of invasion by zebra mussels far outpaced the spread of quagga mussels both in the United States and in Europe. The estimated lag time between initial introduction and maximal population size is 5 times shorter for D. polymorpha than for D. r. bugensis, which may be an important factor affecting the speed with which this species can spread. Differences in population dynamics may facilitate the increased rates of spread of D. polymorpha relative to D. r. bugensis, especially at smaller spatial scales.