Structural examinations of a biofilm immobilized on small cement ball (CB) particles in a three-phase fluidized bed reactor for a highly concentrated nitrification process were carried out. With transmission and scanning electron microscopes, a diverse microbial arrangement was determined. The scanning electron micrographs showed that a large number of clustering bacteria was covered by extracellular polymers. Between the clusters, many open spaces were found. The transmission electron micrographs showed a regular arrangement of bacteria inside the biofilm. Most clusters of Nitrosomonas sp.-like bacteria were found near the surface of the biomm and the majority of Nitrobacter sp.-like bacteria were observed inside the biofilm. At least 14 types of bacteria were found in the mixed culture nitrifying biofilms.