The southern masked chafer,Cyclocephala immaculataOlivier, and the northern masked chafer,C. borealisArrow, are serious pests of turfgrass in central Kentucky. The temperature threshold for pupation of overwinteringC. immaculatagrubs was determined experimentally as 10.8°C. Trapping studies during 1979 and 1980 indicated that first emergence and peak flight of males occur approximately 1 to 2 weeks earlier forC. borealisthan forC. immaculata. Thermal unit accumulations in air and soil were closely correlated with the first emergence of beetles, but they were less useful for predicting the date of 50 and 90% flight. Once emergence has begun, activity of masked chafers is more closely related to rainfall patterns than to temperature.