Istanbul, which is the largest megacity of Turkey with a population over 12 million, tries to solve encountered environmental problems such as severe coastal erosion, shoreline recession and over pollution has been exposed mainly last 20 years due to response of the coast to human activities. The aim of this study was to investigate the bacteriological water quality and occurrence of enteric bacteria of surface waters on the southwestern coast of Istanbul by means of GIS. Seven sampling stations were chosen to conduct measurements on a monthly basis during the year of 2008. The physicochemical parameters were measured in-situ at all sampling stations and to determine the density of the species in enteric bacteria, the DAFOR scale was used. During the spring and summer period, the bacterial density was reduced. From September to November, a unique genus is generally observed in all stations through seawaters getting cold and effect of the streams. However, when the environmental factors started to change, such as temperature increase, different genus were observed. In January, central density in bacteria values has been determined to be only at 5(th) station. Similarly, in the rainy seasons, being added to the washed solid inputs, the seasonal genus appeared. At the stations 1 and 2, the density of the enteric bacteria has been determined as the higher values almost every months except cold season, due to negative effects of anthropogenic pollutant carried by different streams. The results reveal that the study area faces bacteriological pollution and the existing pollution level in this area is above the criterion specified for aquaculture, fishery and recreational activity. Employing modem tools like GIS helps us better understand the spatial distribution of enteric bacteria of sea. These tools might also be used as the initial steps of determining and providing the input data for water quality modeling studies.