The characteristics of sponge attached biomass was investigated. The biomass was developed in an experimental full scale down-flow hanging sponge (DHS) reactor treating 5,000m³/d of sewage, which was built at a sewage treatment plant in Agra, India. Comparing to normal sponge biomass that was fed with Japanese sewage, the sponge attached biomass developed in Agra DHS plant was very stiff and argillaceous with about 100 g-DS/L-sponge of sludge concentration. The biomass was covered with microbial sulfur mat mainly consisting of Thiothrix. sp. In addition, almost 60% of the sludge consisted of inorganic matter. As a result of analysis by X-ray diffraction and X-ray fluorimeter, the inorganics was identified to be kaolins. The sewage treatment and water permeability of the sponge were also tested. As a result, it was suggested that the water permeability to the sponge attached biomass was inhibited by accumulation of kaolins, adversely affecting the sewage treatability of the biomass.