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Periodic algal blooms of three diatom species such as Coscinodiscus centralis, Rhizosolenia alata and Rhizosolenia imbricata were observed during September 2011, December 2011 and March 2012 in the coastal waters of South Andaman Sea at Junglighat bay area (11° 39 N and 92° 43 E). The blooms were intense, with Coscinodiscus at a concentration of 89,000 cells mL-1 (contributing 85 to 98 % to the total phytoplankton population), Rhizosolenia alata at a concentration of 13,000 cells mL-1 (86-93 %) and Rhizosolenia imbricata at a concentration of 19,000 cells. mL-1 (91-99 %). Nutrients, most importantly nitrate and silicate have emerged as the key factors controlling phytoplankton growth in this area.