Species diversity, density and seasonal abundance of polychaetes associated with Sargassum sp in Kudankulam water was investigated for a period of one year from May 2003 to April 2004. A total of 13 species belonging to 6 families were recorded. Platynereis dumerili and Perinereis nigropunetata were the abundant species. Seasonal variation in density and species composition was observed. During the study period the Shannon-Weiner diversity index (H1) ranged from 0.178 to 0.669 and the evenness index (j1) varied between 0.373 and 0.98
