AbstractThe distribution and biodiversity of the phytoplankton in Nansha Islands waters were investigated in spring and summer,1999. The oplankton abunda nce at Zhubi Reef (2.49×10⁴ cells/m³) is higher than that in the open sea (0.73×10⁴ cells/m³ in spring and 1.03×10⁴ cells/m³ in summer, respectively). The order of the biodiversity is spring(4.01)>summer(4.35)>coral reef (2.92). The distribution of phytoplankton in waters around Nansha Islands appears to relate to hydrological conditions, nutrients and grazing impact of zooplankton. In spring, grazing pressure of zooplankton is higher and has an obvious effect on algal distribution; while in summer, phytoplankton biomass is mainly affected by the distribution of local nutrients and hydrological conditions. Although the phytoplankton abundance in 1999 is very low compared with historic record, the characteristics of biodiversity and distribution of phytoplankt on in Nansha Islands waters have been similar in the past 15 years.