A detailed limnological study of Clark's Borrow Pit Pond (SW* Sec. 30 T22N R9E, Delaware County, IN) was conducted from the spring of 1978 through the fall of 1979 to ascertain what factors regulate the abundance of Volvox aureus, a major phytoplanktor of the pond. This algal species often dominates the spring and sometimes fall phytoplankton blooms that occur annually, but disappears fran the plankton throughout the remainder of the year.Laboratory growth experiments suggest that the spring peak of V. aureus (105 colonies ml-l in 1978) is most likely induced by the high nutrient levels in the pond (total P = 4.6 µmoles ℓ-1; NO3 - N + NH3 - N = 11.8 Wholes 2-1) and warming temperatures. Competition from other algal species is reduced at this time by the presence of the zooplanktor Da hnia ambigua which effectively grazes all but the V. aureus colonies. Decreased nutrient levels in the summer months (total P = 1.0 µmoles Z-1 N03 - N + NH3 - N = 1.3 µmoles Z-1 ) restrict the abundance of V. aureus, and other algal species (notably blue-greens) dominate. Two of these species, Oscillatoria sp. and Microcystis sp., were shown to exhibit no allelopathic influences on the growth of V. aureus. However, experiments suggested a heat labile inhibitor or inhibitors of V. aureus may periodically affect its growth in the pond. Increasing nutrient levels in the fall sometimes induce a second V. aureus bloom, but this declines rapidly as the water temperature drops below 10° C. Consequently, the abundance of V. aureus in Clark's Pond is influenced mainly by abiotic factors (nutrient availability and water temperature) and only to a lesser degree by biotic factors (grazing and competition).