For repression of bulking, an attempt was tried by addition of immobilized microorganisms to a mixed liquor. This resulted in the temporary increasing of MLSS and as well as the rapid decreasing of TOC. When some of immobilized activated sludge was added to the aeration tank containing bulking sludge, a remarkable decreasing of the S V I was obserded comparing to the reference, under a batch system. Optimal reservation condition of immoblized microorganisms will be required to for unexpected bulking trouble. Under optimal reservation condition, i.e. in tap water lower than 10 °C, the activity higher than 70% of the initial activity was kept throughout six months.