The predatory cladoceran Cercopagis pengoihas extended its range over the past decade from the Ponto-Caspian basin into the Baltic Sea and the Laurentian Great Lakes. This study employs mitochondrial DNA (mtDNA) se- quence analysis to elucidate the invasion corridors exploited during this range expansion. The Caspian and Black Sea populations of C. pengoi exhibit marked genetic divergence, enabling their discrimination as potential sources of the Baltic and Great Lakes populations. The limited mtDNA haplotype diversity detected in the Baltic populations suggests they were founded by a small number of colonists from the Black Sea. The colonization of North America was apparently a result of the transfer of animals from the Baltic Sea in ballast water. Transoceanic shipping activities and canals linking natu- rally segregated watersheds on single continents are provok- ing a massive redistribution of freshwater life. Euryhaline organisms are well positioned to exploit these dispersal cor- ridors, because shipping often occurs via waters of varying salinity. The Ponto-Caspian basin has had a turbulent history of salinity fluctuation and, consequently, many elements of its fauna have evolved broad salinity tolerances (Banarescu 1991; Dumont 1998). Hence, it is not surprising that many prominent invaders such as the hydrozoan Cordylophora caspia, the mollusks Dreissena polymorpha and Dreissena bugensis, the amphipod Echinogammarus ischnus, and the cladoceran Cercopagis pengoiwere derived from this region (MacIsaac et al. in press). Each of these species first ex- tended its distribution into Eastern Europe (Olenin and Lep- pakoski 1999) and subsequently colonized North America (Ricciardi and MacIsaac 2000). Although the general ele- ments of these range extensions are understood, a detailed dispersal history is lacking. It is, for example, unclear wheth- er the range extensions into Europe reflect dispersal from the Black or Caspian Sea. In addition, it is uncertain if North American populations were derived from these source lo- calities or from populations in recently colonized areas such as the lower Rhine River and Baltic Sea (MacIsaac et al. in press).