The results of an investigation carried out during June 2005 to May 2007 on bloom-forming phytoplankton species composition and abundance in the Parangipettai and Coleroon coastal waters （Southeast coast of India） are reported. Air and surface water temperatures （℃） varied from 25.1 to 30.1 and 24.5 to 28.5, respectively, in the former waters and from 25.5 to 31.2 and 25.0 to 29.3 in the latter waters. The respective salinities varied from 6.0 to 28.5 and 5.0 to 33.1 and the respective pH ranged between 7.0 and 8.3 and 7.2 and 8.3. Correspondingly, the dissolved oxygen content varied from 3.1 to 7.5 and 3.1 to 7.9 mgL1 while the light extinction coefficient （LEC） values ranged between 3.1 and 10.1 and 1.8 and 11.0. The content ranges of inorganic nutrients, i.e., nitrate, nitrite, phosphate and silicate （μmolL^-1）, in the Parangipettai and Coleroon coastal waters were： 6.5- 27.0; 1.0- 8.9; 0.1-3.0 and 15.0 -140 and 10.1-23.4; 1.2-8.9; 0.2-3.1 and 55-125 respectively. The chlorophyll α contents in both waters ranged from 2.0-7.5 μgL^-1. Presently, 124 phytoplankton species representing different classes were recorded in the Coleroon coast, viz, Bacillariophyceae （77）; Dinophyceae （19）; Cyanophyceae （15）; Chlorophyceae （10） and Chrysophyceae （3）, whereas 117 phytoplankton species were recorded in the Parangipettai coast, viz, Bacillariophyceae （66）; Dinophyceae （22）; Cyanophyceae （19）; Chlorophyceae （7） and Chrysophyceae （3）. The phytoplankton cell abundance in the Parangipettai and Coleroon coastal waters varied from 290 to 111662 and 140 to 132 757 cells L^-1, respectively, with peak diversity （3.38 and 3.52 bits indl.） recorded in summer. The maximum abundance occurred in summer coinciding with the stable hydrographical conditions. The seasonal distribution and abundance of phytoplankton are discussed in relation to hydrographical parameters. Totally 31 and 24 species of phytoplankton were found to be bloom-forming in the Parangipettai and Colero
K. Thillai RajasekarM. RajkumarSUN JunV. Ashok PrabuE Perumal
