The heart rate and feeding behavior of one-day and 12-day-old Daphnia magna were studied at eight pH values (4-11) and eight salinities (1-8). The effects of pH and salinity on two ages of Daphnia magna exhibited the similar tendency——to promote at low and to suppress at high. The adaptability to alkaline water is greater than that to acidic water. The optimum requirement of young and old Daphnia magna were 6-8 and 7-9 for pH, 2 and 3 for salinity, respectively. The metabolism of young individuals was higher than the old, and the young were more sensitive to pH and salinity change, but less adaptable to adverse external environment. The heart rate of the young Daphnia at pH 10 and at salinity 5 was (372.1±15.1) and (365.3±10.2) with highly significant difference from the control (P<0.01), higher than the old but narrower adaptability. At pH 8, the filtration rate and feeding rate of the young Daphnia reached maximum and the feeding rate was significantly higher than the control (P<0.05), just as the regularity of the old at pH 9. At the salinity of 2, the filtration rate and feeding rate of young population were the highest, both significantly higher than the control, similar to the old at the salinity of 3. It can be concluded that different ages of Daphnia magna have different optimum pH and salinity, so pH and salinity should be reasonably adjusted to maximize the production.