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SUMMARY
Cohorts of Daphnia pulex from Boskop Dam, an irrigation reservoir near Potchefstroom, Transvaal, were kept at natural food levels under constant temperatures of 10, 15 and 20°C as well as under conditions of temperature with a diurnal fluctuation varying between 17 and 23°C. Daily records were kept of survival, egg production and hatching of young. Life tables were compiled and the intrinsic rate of natural increase calculated. Under conditions of fluctuating temperature (17 – 23°C) the Daphnia population increased by 17,95% per day, whilst under constant temperatures the increases calculated were 12,07% for 20°C, 0,95% for 15°C and 1,44% for 10°C. Juvenile mortalities occurred in all the constant temperature experiments but not in the fluctuating temperature experiment. An inverse relationship was found to exist between cohort life span and increase in temperature.