In the present study, the histological examination of the hepatopancreas (digestive gland) and gills of black mussels, Mytilus galloprovincialis was performed as a part of the biomonitoring programs carried out to assess the biological effects of the Izmir Bay (Western coast of Turkey). Mussels collected from five stations; Urla in the outer Bay and Inciralti, Goztepe, Konak, Pasaport in the inner Bay of Izmir Bay. While normal histology on hepatopancreas tissue was observed in Urla samples, vacuolar degenerations and hemocytic infiltrations were abundant in hepatopancreas throughout the other four stations. Also breakdown of digestive epithelium was determined in only Pasaport station. In gill tissues, while hemocytic infiltration was observed in Urla samples, gradually increasing cilia erosion, fusion, cell loss and necrosis was observed in Inciralti, Goztepe, Konak, Pasaport samples. Lumen was determined as normal in Urla and Goztepe samples and enlarged in Inciralti samples. The lumen in each filament was thinner and in some parts adjoined to each other and in Konak and Pasaport samples. Also the gills abfrontal tissue integrity was disturbed in these two stations.