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The exotic zebra mussel, Dreifrena polymorpha, is an unwelcome invader of North American freshwater systems. This species was discovered in Lake St. Clair in 1988, where it presumably was established from discharges of ship ballast water in 1986 (Miller et al. 1992; Leach 1993). The freshwater ballast, obtained in an European port, probably discharged veligers which thrived in food-rich Lakes Erie and St. Clair. The zebra mussel spread throughout the Laurentian Great Lakes in the United States and Canada, and into 11 States in the next 4 years (Figure 2). It has been detected in the Mississippi River above and below the mouth of the Missouri River, in the Arkansas River, and has probably found its way into the Missouri River (although undetected as yet). Zebra mussels are expected to spread in the Mississippi River, where their dispersal has been similar to that documented for the Great Lakes (Tucker et al. 1993). F/GUi7E Z. ZEBRA MUSSEL S/GHTINGS. Native to the Caspian region of western Russia, the zebra mussel historically inhabited the Aral. Caspian, Azov, and Black seas. The species also was a common inhabitant of the mouths of large rivers that flow into these saline basins (Karnaukhov and Karnaukhov 1993). The zebra mussel began increasing its range in the 19th century by invading North European rivers that emptied into the Black Sea, presumably in response to canal building and increased eutrophi- cation (Karanaukhov and Karanaukhov 1993). Europeans have experienced its continuing invasion through shipping canals for nearly 200 years (and more recently in association with hydroelec- tric projects). By about 1930, the mussel had spread throughout Europe and had reached Great Britain. Scientists have speculated (since about 1921) that zebra mussels might be transmitted to the United States by shipping (Nalepa and Schlosser 1993), but little was done to reduce the likelihood of such an invasion.