Qualitative seasonal distributions of sulfate-reducing bacteria, colorless sulfur bacteria, nitrifying and denitrifying bacteria in the water and sediment samples in the inner part of İzmir Bay were investigated in relation to certain physico-chemical parameters (pH, oxygen, hydrogen sulfide, nitrate, nitrite, ammonium). Denitrifying bacteria and sulfate-reducing bacteria were present in the sediment and even in the aerobic water samples. Hydrogen sulfide was found almost in all water samples measured but it was at lower concentrations. It is suggested that denitrifying bacteria and sulfate reducing bacteria which produce hydrogen sulfide are involved in the mineralization of organic matter. Members of Thiobacillus and Beggiatoaceae were present in all sediment samples. Thiobacillus was found only at a few depths in the water column. Nitrifying bacteria were found in the sediment and in both aerobic and anaerobic water samples. Nitrite did not accumulate in this eutrophic environment, indicating that it was oxidized to nitrate by nitrifying bacteria.