Biological nitrogen removal of Wastewater Treatment Plants (WWTPs) is assumed by Nitrifiers and Denitrifiers in activated sludge. In previous research, Nitrospira, one of nitrifiers, is detected as dominant and key players in WWTPs. However, the representative Nitrospira strains from activated sludge were limited. In our previous progress, it was revealed that isolated Nitrospira japonica strain NJ1 possesses Quorum Sensing (QS) systems. Although QS systems of Nitrifiers was reported by several research teams, there is no report that Nitrospira QS systems are elucidated. In this research, we try to explore the function of QS systems in Nitrospira japonica by transcriptional analysis.