Lake Xinghu is a typical eutrophic lake consisting of 6 sub-lakes in tropical Guangdong Province.We reported the fatty acid composition of phytopJankton community and the relationship between the fatty acids and phytoplankton in the two sub-lakes: Xiannii Lake and Zhongxin Lake in July, October and December of 2002.For the two sampled sub-lakes, Zhongxin Lake had a higher trophic level than the Xiannii Lake.In the former, Cyanophyceans were the dominant phytoplankton in July and October, but was replaced by Chrysophyceans in December.In Jhe latter, Cyanophyceans predominated numerically in all sampling periods.The carbon chains of the fatty acids for phytoplankton in the water samples ranged from 16 to 22.3 species of saturated fatty acids, 3 monounsaturated fatty acids and 7 polyunsaturated falty acids were detected.The saturated fatty acids dominated in the total fatty acids, the polyunsaturated fatty acids were low m the iwo sub-lakes.For single species of fatty acids, the concentration of C16:0 was the highest in all the fatty acids.It showed a significant relationship with the abundance of Cyanophyceans, which indicated that the C16:0 was rooted in the Cyanophyceans.The Bacillariopbeans and Cryptophyceans were rich in EPA, and the concentration of EPA in phytoplankton was high with the biomass of the BacillariopKeans and Cryptophyceans.The high concentration of DHA was observed in Zhongxin Lake in the December, which should be primarilyfrom Chrysophyceans.Generally speaking, the composition of the fatty acids in the water samples was consistentwith the structures of phytoplankton community.