Recent studies of plankton community structure in Adirondack lakes are summarized in relation to lake acidity status. Species richness of phytoplankton, planktonic rotifers, and crustacean zooplankton declines with decreasing pH. Highly acidic waters (pH \< 5.0) average fewer than 20 phytoplankton and fewer than 12 zooplankton species in midsummer collections. More circumneutral lakes (pH \> 6.0) average more than 33 phytoplankton and 20 zooplankton species. This decline in species number represents a simplification of plankton community structure rather than the invasion of new species. Very few Adirondack plankton species can be considered alkalibiontic or acidobiontic. The dominant species of acidic lakes are also generally important in non-acidic lakes of the region. Thus, presence or absence is generally not a very good indicator of lake acidity. However, shifts in the relative abundance/biomass of some species or species groups appear to be consistent with changes in pH. The relative biomass of acidobiontic diatoms and mallomonads is reported to provide a good index of acidity status. The relative biomass of the rotifer, Keratella taurocephala provides a consistent indicator of acidity. Among the crustacean zooplankton it appears that the combined relative biomass of Diaptomus minutus and Bosmina longirostris may be a useful indicator of water quality changes associated with acidification.