Field sampling along line transects in Raquette Lake, the largest natural lake in the Adirondack Mountains, enabled comparison of the flora encountered in 1983 with that recorded in a 1933 survey, and documentation of quantitative vegetation change between 1983 and 1999/2000. More than 30 submersed and floating-leaved macrophyte species were found in 1933 and 1983, a relatively high species richness for a lake. Nine species not seen in 1933 were found in 1983, and an additional two species by 2000, indicating considerable species turnover. The most important species at two sites in 1983 declined sharply by the end of the century, while the inflated bladderwort (Utricularia inflata) rose from not being present to become the most frequent species at both sites. This bladderwort, only recently reported north of New Jersey and only at sites more than 250 km distant from Raquette Lake, may have contributed to the decline of other species. Utricularia inflata appears to be expanding its range rapidly: it has also appeared at one other site in Raquette Lake and at sites in Limekiln Lake and the Fulton Chain of Lakes (Seventh Lake) in the Adirondack Mountains. This species has the potential to cause substantial ecological change.