This study provides baseline data on the zooplankton community of LaCygne Lake before its use as a cooling pond and documents changes occurring after startup of the electrical power generation station. The spring zooplankton maximum shifted from July during the summer before plant operation to May and April in the two successive years after plant operation began. Zooplankton density increased significantly after the plant began operation. In situ thermal toler- ance experiments simulated passage through the condenser and discharge canal by exposing zoo- plankton 4 h to temperatures ranging from 35° to 40°C. Bosmina longirostris was the most heat resistant species. B. longirostris increased in density more than other zooplankton species after plant startup. Reservoirs are increasingly being used as cooling ponds to dissipate heat produced during the generation of electrical power. Use of a lake as a cooling pond can potentially affect the lake zooplankton community in many ways. The plant may cause zooplankton mortality when water from the cooling pond is pumped through the plant to remove heat from the steam condensers. Water emerging from the condenser is usually heated from 6 to 17°C above ambient lake temperatures (Clark, 1969) and may ultimately reach tempera- tures above zooplankton thermal tolerances, favoring population growth of heat resistant species. Another effect is that the discharge of the heated con- denser effluent back into the cooling pond may increase lake temperatures and alter zooplankton succession which is, in part, temperature dependent (Hall, 1964; Hutchinson, 1967). Most research related to the zooplankton of cooling ponds has focused on zooplankton thermal tolerance or mortality caused by condenser entrainment (Finesinger, 1926; Brown and Crozier, 1927; Brown, 1929; Heinle, 1969; Goss and Bunting, 1976; Carlson, 1974; Storr, 1974; Buikema et al., 1978). Few stu- dies have examined the combined impact of condenser passage and lake heat- ing on the zooplankton community of the cooling pond (Carlson, 1974; Krza- nowski, 1979). This study establishes baseline data on LaCygne Lake before it was used as a cooling pond and investigates initial changes in the zoo- plankton community of LaCygne Lake following electrical generation startup.