This study was carried out with the aim of determining the Ecological Quality Status (EQS) of coastal transitional waters of Susurluk River Basin (Marmara Sea). Macrozoobenthic invertebrate material was collected in May of 2013 from six stations in triplicate by means of a van Veen grab. Shannon-Weaver Diversity Index and associated Pielou's Evenness Index together with biotic indices AMBI and BENTIX were calculated. Some ecological variables (Secchi disc depth, temperature, salinity, dissolved oxygen, pH, total phosphorus, total nitrogen, silica, Chlorophyll-a) of the sea water and total organic carbon of the sediment were also measured. Ecological Quality Status at each station was assessed by considering individual dominancy of sensitive and tolerant species, results from univariate/biotic indexes and some ecological parameters. When EQS of the basin is evaluated as a whole, it was seen that there was not any station in "Good" or "High" EQS at the study area, stations were in "Poor" or "Bad" EQS at the stations where the River meets the sea, while the stations off the River mouth were in "Moderate" EQS. Both species pattern of macrozoobenthic invertebrate communities and ecological parameters determined in the basin such as TOC and nutrients present the negative impacts of the River over the basin and indicate the necessity of efficient precautions to be taken.