Oneida Lake is a eutrophic lake, located in the low, open relief of the Ontario Lake Plain in central New York state. A 1992–95 survey of the molluscan community produced 21 living species, which represents a 50% reduction in species richness over the past 70 years. Of these 21 species, six have been introduced since 1917; the most recent invader, the zebra mussel (Dreissena polymorpha) is present in numbers that exceed all other molluscan species combined. Since water quality has generally remained consistent over the years, the observed changes in the historic molluscan fauna are probably the result of biotic interactions with introduced species as well as habitat loss due to human modifications of the eulittoral environment.