Background/Question/Methods The New Zealand mud snail (Potamopyrgus antipodarum) is a global invasive species that has been present in the Great Lakes since, at least, 1991. The snail has been shown to cause substantial ecological changes to its freshwater habitat in many places where it has invaded. However, the impact of the snail on the Great Lakes region is poorly known. Several locations in Lake Ontario and Lake Erie were examined for the presence of New Zealand mud snails using a benthic grab. Samples were taken at depths ranging from 1m to 45 m. Samples were also taken from several streams emptying into Lake Ontario. An experiment was also conducted to examine the effect of salt concentration on the growth of invasive and native clones of the snail. Results/Conclusions The snail was originally discovered in Lake Ontario, but its range has since expanded within Lake Ontario and to include four of the five Laurentian Great Lakes. It has also recently spread into flowing waters in the Lake Ontario watershed. Within Lakes Ontario and Erie, its distribution appears to be influenced by depth as densities of the snail peak between 15 and 25 m and it is rarely found in water of less than 5 m in depth. Data from the experiment shows that invasive clones tolerate variation in salinity better than clones from the native range in New Zealand.