Seasonal dynamics of soft-bottom polychaetes were investigated at 29 stations (depths: 8-77 m) in Izmir Bay (Aegean Sea. eastern Mediterranean) between 1997 and 2002. A total of 396 species belonging to 44 families were encountered, of which 32 species were new records for the Turkish fauna. Diversity and evenness index values were high at stations located in the outer part of the bay, whereas azoic conditions Occurred in the Polluted inner part of the bay, particularly in summer and autumn (fall). The highest population density was recorded at station 29 (inner part). with 11620 ind. m(-2). where Capitella capitata capitata was the most dominant species (6820 ind. m(-2)). Stations located in the semi-polluted zone of the bay were generally characterized by high biomass values (max. 33.6 m(-2)) The cluster and nMDS analysis showed that the soft-bottom of the bay has four major polychaete communities, mainly structured by prevailing environmental conditions and types of substratum.