The present study reports the occurrence of two exotic species of polychaetes in Izmir Bay, the eastern Mediterranean: Streblospio gynobranchiata, newly reported for the Mediterranean, and Polydora cornuta. Their establishment in the area is most probably transport by ballast water since these two species greatly dominated polluted muddy bottoms near and in Alsancak Harbour. The maximum density and biomass of S. gynobranchiata in the area were 34,270 ind m−2 and 9·4 g wwt m−2, and those of P. cornuta were 3170 ind m−2 and 2·1 g wwt m−2, respectively. These two species accounted for more than 60% of total faunal populations in the majority of samples collected in winter. Both species are capable of reproducing in the area both in summer and winter. The morphological, ecological and distributional characteristics of these species as well as their impacts on the prevailing ecosystem are analysed and discussed.