Abstract: This research has been conducted to determine seasonal changes of zooplanktonic organisms (Rotifera, Cladocera, Copepoda) in the lake Terkos. Zooplankton samples have been collected in five different stations in monthly periods between April 2000-March 2001. As a result, 1992556 ind m-3 was found in lake Terkos and it is made up of 1177334 ind m-3 Rotifera (59.1%), 117583 ind m-3 Cladocera (5.9%) and 697639 ind m-3 Copepoda (35%)