This paper reports the results of estimations of production and respiration of planktonic Protozoa in lake Donghu, Wuhan. The experiments were carried out during Match 1987-Match 1988. Data of protozoan generation time used in calculation were obtained in laboratory culture and field observation. Within the range of water temperature 6-28 ℃, statistics showed that the protozoan generation time (G) has a linear correlation organism size (V) and water temperature (T): logG(hr) = 0.818 + 0.216logV(um~3) - 0.027T(℃) (P 0.05) During 1987-1988, the mean daily production(dry weight) of Protozoa at two stations in the Lake are 95.0mg/m~3/d(station I) and 73.8mg/m~3/d(station II) respectively. with the gradually accumulation of nutrients in the Lake, to compare 1963, the mean daily production of Protozoa has been increasing rapidly, from 32.25 to 95.0mg/m~3/d at station I and from 34.14 to 73.8mg/m~3/d at station II. The measurement of protozoan respiration was carried out with Gilson Differential Respirator. The protozoanrespiation rates at station I and II are 104.45 and 73.2 ml/m~3/d respectively. According to the data of production and respiration of Protozoa, the protozoan net production efficiencies at two stations are caculated. They are 48.08% (station I) and 53.66% (station II). The average annual P/B ratior is 350 at station I and 340 at station II. In the paper the production and net production efficiency of Protozoa are compared with those of other aquatic animals of the lake. It showed that production of Protozoa is greater than the total production of other zooplanktons(Rotifera, Cladocera and copepoda). Based on the relationship of the primary production and secondary production, the effeciency of Protozoa and algae is 5.52-9.85% which is larger than that of the other aquatic animals. The results also showed that the eutro phication in Donghu Lake becomes more and more serious. The roles of Protozoa in aquatic eco system are analysed.