Profundal sediments of Lake Constance were sampled at 60^130 m water depth and studied with respect to the population sizes and depth distribution of mixotrophic and lithotrophic iron-oxidizing, denitrifying bacteria. The sediment cores showed a distinct stratification with oxygen and nitrate being consumed within the first centimeter of the sediment. The sediment contained large amounts of acid-extractable iron (0.8^1.4% of the dry mass). The proportion of ferrous iron in the solid phase increased dramatically from 16^35% in the oxidized surface layer to 79^97% below 4-cm depth, whereas the total iron content increased only slightly with depth. The concentration of dissolved ferrous iron in the pore water ranged between 10 and 80 WM, which was about three orders of magnitude lower than the total iron content of the solid phase. The redox profile of the pore water iron followed that of the solid phase. Most probable numbers (MPNs) of acetateoxidizing, denitrifying bacteria in the different cores ranged from 1.0U10 6 to 2.1U10 8 cells (ml sediment) 31 ; highest numbers were found in the upper sediment layer. In most cores, bacteria capable of iron oxidation made up a significant part (1^58%) of the total cultivable denitrifying population. Between 1.0U10 4 and 5.8U10 5 cells ml 31 oxidized iron mixotrophically; the numbers of lithotrophic ironoxidizing denitrifiers were about one order of magnitude lower. MPNs of mixotrophic iron oxidizers and aerobic iron oxidizers among the denitrifying populations in the upper sediment layers were in the same range. Generally, the MPNs of the different iron-oxidizing populations did not change significantly with depth. ß