The succession of sulfur-oxidizing bacterial (SOB) community structure and the complex internal sulfur cycle occurring in wastewater biofilms growing under microaerophilic conditions was analyzed by using a polyphasic approach that employed 16S rRNA gene-cloning analysis combined with fluorescence in situ hybridization, microelectrode measurements, and standard batch and reactor experiments. A complete sulfur cycle was established via S 0 accumulation within 80 days in the biofilms in replicate. This development was generally split into two phases, (i) a sulfur-accumulating phase and (ii) a sulfate-producing phase. In the first phase (until about 40 days), since the sulfide production rate (sulfate-reducing activity) exceeded the maximum sulfide-oxidizing capacity of SOB in the biofilms, H 2 S was only partially oxidized to S 0 by mainly Thiomicrospira denitirificans with NO 3 ؊ as an electron acceptor, leading to significant accumulation of S 0 in the biofilms. In the second phase, the SOB populations developed further and diversified with time. In particular, S 0 accumulation promoted the growth of a novel strain, strain SO07, which predominantly carried out the oxidation of S 0 to SO 4 2؊ under oxic conditions, and Thiothrix sp. strain CT3. In situ hybridization analysis revealed that the dense populations of Thiothrix (ca. 10 9 cells cm ؊3 ) and strain SO07 (ca. 10 8 cells cm ؊3 ) were found at the sulfur-rich surface (100 m), while the population of Thiomicrospira denitirificans was distributed throughout the biofilms with a density of ca. 10 7 to 10 8 cells cm ؊3 . Microelectrode measurements revealed that active sulfide-oxidizing zones overlapped the spatial distributions of different phylogenetic SOB groups in the biofilms. As a consequence, the sulfide-oxidizing capacities of the biofilms became high enough to completely oxidize all H 2 S produced by SRB to SO 4 2؊ in the second phase, indicating establishment of the complete sulfur cycle in the biofilms.In wastewater biofilms, due to the relatively high organic input and low dissolved oxygen (DO) concentration, an internal sulfur cycle consisting of sulfate reduction and subsequent sulfide oxidation is an important process for carrying electrons from the deeper anoxic zone to the oxic surface zone. Consequently, the sulfur cycle could be responsible for mineralization of a substantial part of the organic matter and consumption of dissolved oxygen (19,25,34), which prevent the emission of odorous and toxic hydrogen sulfide gas from the biofilms.The reductive side of the sulfur cycle (i.e., sulfate reduction) occurs only biologically. Thus, the population dynamics, biodiversity, and in situ ecophysiology of sulfate-reducing bacteria (SRB) in wastewater biofilms have been extensively investigated by combining a 16S rRNA gene approach and microelectrode measurements (11,12,25,26,27,34). In contrast, the oxidative side of the sulfur cycle (i.e., sulfide oxidation) occurs both biologically and chemically. The chemical oxidation reaction is, however, rather s...