We previously reported that partial nitrification in the down-flow hanging sponge (DHS) system was satisfactorily accomplished under oxygen-limited conditions [Chuang et al, Water Res., 41, 295-302 (2007)]. In this study, we investigated the microbes that are responsible for the partial nitrification in this unique system by 16S rRNA- and amoA-based cloning analyses and fluorescence in situ hybridization. Microbes related to Nitrosomonas species were found to be chiefly responsible for catalyzing the partial nitrification. Microbes affiliated with the uncultivated phyla OP10 and Bacteroidetes were also numerous in the DHS, but their ecological niches are still unknown.