Ecological evaluation of the impact of an exotic species upon native plant species is frequently a combination of historical data prior to introduction and after full establishment with little observation in between. The introduction of Myriophyllum spicatum L. (Eurasian watermilfoil) into Lake George, New York, U.S.A. was first noted in 1985. In 1987, a few newly established plants were allowed to grow unimpeded by human management to document the rate of colonization of this species into a new habitat with its ultimate dominance over, and systematic elimination of, native species. This changing community has been closely monitored over the past decade. Initially a 6 m2 grid system composed of 144 0.5 m2 plots with four radiating transect lines was laid out with the isolated M. spicatum plants as the center. The site was revisited between 1987 and 1997 to mark the extent of the developing M. spicatum bed and its effect on the composition of the native plant community. Since 1987, the area of dense growth has expanded in all directions, impeded only where some physical barrier, such as upper or lower water depth limits or sediment type curtailed its growth. Concurrently, with this expansion, a decline in species richness and abundance of native species was observed.