1. Digestive enzyme activity and the oxygen consumption rate of Acartia clausi from the polluted eutrophic Elefsis Bay and the non-polluted oligotrophic Vouliagmeni-Fleves area were compared. 2. Out of 13 hydrolases whose activity was revealed, 10 had a higher activity in the Elefsis population than in the Vouliagmeni-Fleves one. Only amylase, alkaline phosphatase and esterase lipase had a higher activity in the Vouliagmeni-Fleves population. 3. The high activity of amylase in Vouliagmeni-Fleves population seems to indicate that phytoplankton constitutes the most important part of A. clausi's food. 4. Respiration rate was significantly lower in Elefsis population, irrespective of season. 5. A significant seasonal variation was observed which was greater in the Vouliagmeni-Fleves population, with a 50% decrease in the respiration rate between spring and summer. 6. The obtained results agree with previous literature data and show the physiological characteristics of A. clausi of Elefsis Bay, allowing it to thrive in polluted eutrophic waters.