Conjoined twinning is a rarely existed gross malformation in natural conditions. The term refers to the two individuals united at any part of their bodies. The extraordinary incidences of conjoined twinning cases may reflect important problems on the marine environment. Therefore, the purposes of this study were to report the incidences of Trachurus mediterraneus conjoined twins in Bandirma Bay, Southern Marmara Sea and to discuss the possible causes of them to derive new research questions. Four polyembryonic individuals were collected from three stations of Bandirma Bay Ichthyoplankton Survey in June 2012. The anomalous individuals were observed at the stations, St-9, 10 and 13 (Critical Stations). St-9 was at about 20m depth contour between Karsiyaka town and Dalyan village in western coasts of bay. St-10 and 13 was off Yenice village at about 10 and 30m depth contours respectively in the eastern coasts of the bay. The surface temperature, salinity, dissolved oxygen and pH of critical stations were seemed to be normal. Therefore the pollution was considered as the most possible reason of these incidences.