A study of zooplankton dynamics was carried out in Lake Mendota, Wisconsin, for the years 1976–80. The data were compared with those of E. A. Birge, obtained in the years 1894–96. 2The zooplankton annual cycles and sizes of standing crop were similar in both studies. There was more year-to-year variation within each study than there was between the two studies separated by over 80 years. 3The data are discussed in relation to the prediction of long-term change in lakes. The value of a historical data set for long-term ecological studies is emphasized.