This study aims to determine the environmental factors affecting the fauna and distribution of benthic amphipod species inhabiting in the Bandirma and Erdek Gulfs which are located on the south of the Marmara Sea. Total of 66 species belonging to 22 families were identified after analyzing the samples collected seasonally from the depths ranging between 1 and 30 m between 2007-2008. According to the data gathered from the literature, it was determined that one species (Bathyporeia elegans Watkin, 1938) was a new record for the Turkish seas and 37 species for the Marmara Sea. In the Bandirma Gulf, of the ecological variables of the environment temperature was determined to range between 6.6-27 degrees C, salinity between 21.32-36.03 psu, dissolved oxygen between 4.04-11.26 mg l(-1) and pH between 8.00-8.38. In the Erdek Gulf, temperature ranged between 6.7 and 27 degrees C, salinity between 21.93-35.54%, dissolved oxygen between 3.67-13.26 mg l(-1) and pH between 8.06-8.36. In the surface sediment at the sampling stations of the Bandirma Gulf, total organic carbon values were between 0.07-4.42%, total calcium carbonate between 0.88-84.82%, total phosphorus between 609-12740 mu g g(-1) and mud percentage between 1.38-79.93%. In the Erdek Gulf, total organic carbon ranged between 0.08-2.89%, total calcium carbonate between 0.74-80.06%, total phosphorus between 376-3614 mu g g(-1) and mud percentage between 2.12-95.65%.