The study on Polycheata community structure was carried out in the Tam Giang-Cau Hai lagoon, Thua Thien Hue province, during March and December 2006. A total of 36 samples station were collected. With total of 49 species belonging to 20 families, the Polycheata were relative diversity. Among these family Glyceridae with 6 species, families Nereidae, Nephtyidae with 5 and 4 respectively
other families were only 1-2 species. There are two species, viz. Pseudocapitella sp. and Ophiuricola cymip occurred through out seasons with high density with 118 and 134 individual/m2, respectively, whereas species Notomastus sp. was occurred in the transitional time (August) with 80 individual/m2 and species Prionospio poIybranchiata was occurred only in dry season (May) with 64 individuallm2, other ones were fluctualable in between 20-30 individual/m2. Biomass of Polycheata in the rain season was estimated with 748mg/m2 that were higher than in dry season (483.2mg/m ) and transitional time (491.8mg/m2). The community structure of Polycheata is depending on the water temperature, salinity and food resources in the sediment.