Oil and oil-dispersant mixtures at nominal concentrations of 100 and 20 ppm, respectively, were added to a series of ponds constructed for the study. In the pond treated with oil, there were no discernible short or long term effects on the phytoplankton. Both oil-dispersant treated ponds exhibited fluctuations in the dominant class of algae while the concentration of oil was greater than in the water column. Once the oil concentrations were below this value, there was no apparent effect. Periphytic material on the sides and bottoms of the oiled and control ponds were similar in mass and composed of a large number of species. Periphyton biomass was at least three times greater in both oil-dispersant-treated ponds with one genus dominating the growth. These conditions persisted in one of the oil-dispersant-treated ponds one year after treatment but those in the other pond had decreased to levels in the control pond at this time. Dissolved oxygen (DO) values decreased to about 4.6 ppm in both oil-dispersant-treated ponds shortly after treatment, but remained at approximately the saturation level in the other ponds for 6 weeks after treatment. Then these lower values gradually increased until they were slightly greater than in the controls. During the late winter months, both oil-dispersant ponds had anoxic zones above the sediment, the extent depending on the contours of the bottom. The DO values in the other ponds were at the saturation level. In the early spring, the nitrate ion-concentrations in control and oil-treated ponds were twice those measured for the oil-dispersant ponds. No discernible differences dependent on treatment were observed for nutrients or other ions monitored on a regular basis.