Abstract: We reported the composition of phytoplankton fatty acids and the relationships between the fatty acid composition andphytoplankton communities,the fatty acid contents and trophic states in three tropica- l subtropical reservoirs: Liuxihe Reservoir,Feilaixia Reservoir and Gongping Reservoir in the dry season(October and December) of 2002. In three reservoirs, the abun -dances of phytoplankton were low and there were no distinct dominant species but the relative abundance of Bacillariophyceacewere high.The abundance of all phytoplankton in Gongping Reservoir was higher than that in the other two reservoirs, especiallythe abundance of Bacillariopheans was much higher. In theOctober, the trophic states of three reservoirs were different,theLiux- ihe Reservoir was near to oligotrophic,Gongping Reservoir was mesotrophic and Feilaixia Reservoir was oligo -mesotrophic. In theDecember, all the three reservoirs were mesotrophic.The concentrations of fatty acids in the waterwas low,ranging from 11783 to 91808Lg/L. In the October, the fatty acid species were rich and the concentration was high in the Liuxihe Reservoir and FeilaixiaReservoir. EPA and DHA were detected in the two reservoirs, while only saturated fatty acids were detected in the GongpingReservoir. In the December, the composition of fatty acids was simple and only the saturated fatty acids were detected in the threereservoirs with the exception of two species of monounsaturated fatty acids ( C16: 1 and C18: 1) in the Feilaixia Reservoir.Thus,the composition of fatty acids reflected the difference of the trophic states in the three reservoirs. It needs to be pointed out thatonly the saturated fatty acids were detected in Gongping Reservoir in the two sampling periods. Regarding of the high chlorophylla concentration in the reservoir, it may suggest that the phytoplankton without unsaturated fatty acids were difficult to be grazedand utilized effectively by the zooplankton, being accumulated in the water. Generally speaking, the fatty acid was sensitive andfeasible to indicate the change in the phytoplankton community and trophic state of the reservoirs.