Abstract: Najas major community and Najas major+Najas minor community in Lake Donghu were studied during 1962—1964 and 1972—1982.In 1962 and 1963, lake area covered by N. major community was 2.98 km2, while that by N. major+N. minor community was 1.89 km2. For both communities the total area was 4.87 km2, comprising 20.48% of the aquatic vegetation in the lake or 17.09% of the lake area. N. minor has gradually diminished since 1972, while N. major kept on developing during 1976, 1978 and 1979, up to an area of 5 km2 and equivalent to 90% of the total aquatic vegetation area at that period.In 1978 the maximum standing crop of N. major community was 85.75 gm-2 (dry wt.) or 428.73 metric tons (dry wt.) in whole lake, about 2.2 times as that N. major and N. minor summed together during 1962—1963.Life history of N. major has been observed. Every 100g fresh plants produced 91—380 seeds, which germinated in April, with rather high germinative rate.The photosynthesis and dark respiration of N. major has also been estimated. Under laboratory conditions of light intensity of 8,000 lx, water temperature of 28±1℃ and lake water, net production and dark respiration were 0.08±0.03 and 0.12±0.06mg O2 g-1(fresh wt.) h-1, respectively. The P/R ratio was 0.67. The photosynthetic efficiency of N. major in the lake was measured as 0.06% during 1976.According to this study, N. major is a species of low productivity and little economic value. The coarse, spine-tipped teeth on its leaf and stem are also troublesome to swimmers. Therefore, this paper emphasizes the importance of the transformation of this community.