The activities and vertical spatial distribution of sulfate reducing bacteria (SRB) in an oxygen (O-2)-based membrane aerated biofilm (MAB) were investigated using microsensor (O-2 and H2S) measurements and molecular techniques (polymerase chain reaction-denaturing gradient gel electrophoresis [ PCR-DGGE] and fluorescence in situ hybridization [FISH]). The O-2 concentration profile revealed that O-2 penetrated from the bottom (substratum) of the gas permeable membrane, and was gradually consumed within the biofilm until it was completely depleted near the biofilm/bulk liquid interface, indicating oxic and anoxic zone in the MAB. The H2S concentration profile showed that H2S production was found in the upper 285 mu m of the biofilm, indicating a high activity of SRB in this region. The results from DGGE of the PCR-amplified dissimilatory sulfite reductase subunit B (dsrB) gene and FISH showed an uneven spatial distribution of SRB. The maximum SRB biomass was located in the upper biofilm. The information from the molecular analysis can be supplemented with that from microsensor measurements to better understand the microbial community and activity of SRB in the MAB. (C) 2014 Wiley Periodicals, Inc.