The variations of the three phosphorus fractions—inorganic, organic and particulate P have been discussed in relation to the primary production in Gulf of Mannar off Mandapam. Though the rate of primary production is uniformly high, instantaneous concentration of inorganic P is low and without significant seasonal variation. But the total P, dissolved organic P and particulate P show definite seasonal variation. From primary production the rate of phosphate assimilation and regeneration have been deduced.