Abstract: Measurements were made on the chlorophyll contents and trophic states in six connected small lakes in the central area of Beijing city, during January-December 1987. The results on the northern four lakes are presented in this paper. 1. Chlorophyll contents showed marked differences among the lakes, and were much lower in the northern three lakes than in the southern lake, Lake Beihai. 2. In Lake Beihai, chlorophyll contents showed marked seasonal variations, being highst in summer and lowest in winter. The same trend was observed, though to a less extent, in the other three lakes. The results indicated that the trophic level was much higher in Lake Beihai than in the other three lakes. 3. Regression analysis showed that the content of chlorophyll a increase with increased water temperature, water colour, total phosphoius and algal density, and decreased transparency. 4. Evaluation of water quality based on chlorophyll a content suggested that the northern three lakes were mesoeutrophic, and Lake Beihai was eutrophic. The results agreed well with those from a conprehensive evaluation based on several biological, physical and chemical variables. Thus the chlorophyll a content is a satisfactory biological index for the evaluation of trophic level of lakes.