Genetic variability and structure of nonindigenous vs native populations are compared for the Eurasian round goby Neogobius melanostomus and the tubenose goby Proterorhinus marmoratus, which both invaded Lake St. Clair of the North American Great Lakes about 1990. The round goby spread rapidly to all of the Great Lakes and the tubenose goby largely has been restricted to Lake St. Clair, with some recent range extension into western Lake Erie. Risk analyses may indicate whether genetic variability of colonizers is predictive of their relative invasive and establishment successes. The present investigation examined DNA sequence variation across the left domain of the mitochondrial DNA cytochrome b gene in round and tubenose gobies from Eurasian and Great Lakes locations. We also sequenced six additional Neogobius species (including the monkey N. fluviatilis, racer N. gymnotrachelus, and bighead N. kessleri gobies that have been 'on the move' in Europe) and the knout goby Mesogobius batrachocephalus from the Black Sea in order to develop diagnostic genetic characters to identify them in case of future and/or undetected invasions and to delineate their phylogenetic relationships. Results show that a diverse number of haplotypes charac-terize round and tubenose goby populations from both North America and Eurasian sites, fitting a risk analysis prediction of high genetic variability in their successful introductions. Phylogenetic results indicate that the current genus Neogobius is paraphyletic and that the subgenus Apollonia thus should be elevated to the level of genus, containing Apollonia (N.) melanostomus (the round goby) and A. (N.) fluviatilis (the monkey goby). In addition, there appear to be two separate species of Proterorhinus marmoratus, a marine P. marmoratus Pallas 1814 in the Black Sea (matching the original type locality), and a 'cryptic' freshwater species in the Danube and Dnieper Rivers and probably other Eurasian freshwater habitats, as well as invasive in the Great Lakes. We suggest resurrecting the name P. semilunaris Heckel 1837 for the freshwater species (a taxon that was origi-nally described from rivers draining into the Aegean Sea and the Danube River, but was later placed in synonymy with P. marmoratus).