The information presented here is a phase of an overall investigation entitled "Environmental Changes Produced by Cold-water Outlets from Three Arkansas Reservoirs", supported by funds provided by the Office of Water Resources Research, and published in a bulletin by Hoffman and Kilambi (1970). The data herein presented compare the seasonal cycles of net plankton of a natural stream (Kings River) with a new tailwater (Beaver Reservoir, impounded in 1965). Quantitative net plankton abundance and physicochemical conditions at each area were monitored bimonthly from September 1967, through October 1968. Mid-winter blooms of Chrysophyta at the Beaver Dam stations were preceded by an increase in average monthly temperatures and followed by a decrease in silica concentrations. Downstream from Beaver Dam, late summer blooms of Cyanophyta were recorded. Most of these increases occurred in conjunction with high average dissolved oxygen concentrations and temperatures. At the Kings River, winter blooms of Chlorophyta coincided with maximum dissolved oxygen concentrations. Summer blooms appeared to show a positive correlation with increases in temperature and dissolved oxygen concentrations. Zooplankters were most abundant during the winter months at the Beaver Dam stations. Consequent net zooplankton decreases appeared to be inversely proportional to the average seasonal temperature. From July to October 1968, the Kings River stations showed a positive correlation of zooplankton number with seasonal increases in temperatures and decreases in average riffle speeds.