High phytoplankton biomass and ocean primary productivity (OPP) often occurs near river mouths or in eutrophic coastal waters for short time periods.Tang et al.(2004) reported that an increased phytoplankton biomass in a narrow jet-shaped protrusion into the western South China Sea (SCS) using satellite chlorophyll a (chl a) data and concurrent ship measurements in 1999,and also observed this phenomenon during every southwesterly monsoon season from 1997 to 2002.We also found that OPP estimated by the modified size-fractionated primary productivity algorithm (Kameda,2003; Kameda and Ishizaka,2005) in this area occurs in a narrow jet-shaped protrusion or a large anticyclonic gyre into the western SCS from 1998 to 2005.In June from 1998 to 2005,especially in 1999,regional OPP appeared as a large jet shape extending from the coastal waters of Vietnam eastward towards the SCS,about 200 km northeast of the mouth of the Mekong River; this feature intensified in the form of a large jet or gyre from July to September,decayed in October,and disappeared entirely in November.The gyre had OPP about from 250 to 1000 mg C m–2 d –1.This phenomenon may be caused by a strong offshore upwelling often occurring during a period of strong southwesterly winds alongshore,since in the SCS,nutrients are most important factors controlling phytoplankton growth.In addition,the river discharge from Mekong River was also the main source of nutrients in this area.Moreover,the seasonally ocean circulation and currents caused by monsoon maybe also influence the phytoplankton and OPP directly.