Microbial iron oxidation is an integral part of the iron redox cycle in wetlands. Nonetheless, relatively little is known about the composition and ecology of iron-oxidizing communities in the soils and sediments of wetlands. In this study, sediment cores were collected across a freshwater tidal marsh in order to characterize the iron-oxidizing bacteria (FeOB) and to link their distributions to the geochemical properties of the sediments. We applied recently designed 16S rRNA primers targeting Gallionella-related FeOB by using a nested PCR-denaturing gradient gel electrophoresis (DGGE) approach combined with a novel quantitative PCR (qPCR) assay. Gallionella-related FeOB were detected in most of the samples. The diversity and abundance of the putative FeOB were generally higher in the upper 5 to 12 cm of sediment than in deeper sediment and higher in samples collected in April than in those collected in July and October. Oxygen supply by macrofauna appears to be a major force in controlling the spatial and temporal variations in FeOB communities. The higher abundance of Gallionella-related FeOB in April coincided with elevated concentrations of extractable Fe(III) in the sediments. Despite this coincidence, the distributions of FeOB did not exhibit a simple relationship to the redox zonation inferred from the geochemical depth profiles. ; Microbial iron oxidation is an integral part of the iron redox cycle in wetlands. Nonetheless, relatively little is known about the composition and ecology of iron-oxidizing communities in the soils and sediments of wetlands. In this study, sediment cores were collected across a freshwater tidal marsh in order to characterize the iron-oxidizing bacteria (FeOB) and to link their distributions to the geochemical properties of the sediments. We applied recently designed 16S rRNA primers targeting Gallionella-related FeOB by using a nested PCR-denaturing gradient gel electrophoresis (DGGE) approach combined with a novel quantitative PCR (qPCR) assay. Gallionella-related FeOB were detected in most of the samples. The diversity and abundance of the putative FeOB were generally higher in the upper 5 to 12 cm of sediment than in deeper sediment and higher in samples collected in April than in those collected in July and October. Oxygen supply by macrofauna appears to be a major force in controlling the spatial and temporal variations in FeOB communities. The higher abundance of Gallionella-related FeOB in April coincided with elevated concentrations of extractable Fe(III) in the sediments. Despite this coincidence, the distributions of FeOB did not exhibit a simple relationship to the redox zonation inferred from the geochemical depth profiles.