The soft-bottom zoobenthic community structure of Izmir Bay was examined seasonally at eight stations (depth-range: 19-67 m) in 2009. A total of 427 species were found. Polychaeta had the highest number of species (50% of total species) and individuals (7596), whereas Echinodermata possessed the highest biomass (47%). The number of species varied from 3 to 79 (0.1 m(-2)), the density from 60 to 5360 ind.m(-2), and the biomass from 1 to 530 g.m(-2). The most numerically dominant species were the polychaetes Aricidea claudiae, Streblospio gynobranchiata, Levinsenia demiri and Sternaspis scutata. The distribution of zoobenthos was strongly related to spatial differences in total organic carbon, sediment texture and depth among different regions of the bay rather than temporal differences among seasons. However, significant seasonal variability in community structure (mainly differences in the relative abundance of species) was present. The inner region of the bay can be classified as 'poor' or 'bad' based on the results of biotic indices (H', AMBI, m-AMBI and BENTIX). Among biotic indices, only H' and m-AMBI appeared to be capable of explaining the bay's benthic quality status. Thirteen alien species were also found. Streblospio gynobranchiata, Prionospio pulchra, Pseudopolydora paucibranchiata and Polydora cornuta formed dense populations in the inner most polluted part of the bay and are considered to be new pollution indicator species in the eastern Mediterranean Sea.