The possible symbiosis between bacteria and anaerobic archaea was investigated in intermittent aeration (l/A) systems. Archaea solution added to l/A reactor might play an important role in biological activities as well as in improvement of mineralization of organic matter. l/A reactor with archaea solution (l/Aarch) could increase both nitrification and denitrification rate and also reduce the sludge yield remarkably. These results indicate the possibility of the symbiotic activated sludge system with anaerobic archaea by controlling the DO level in the aeration tank. In this study, DO was controlled by intermittent aeration schemes and a successful symbiotic activated sludge system was achieved to reach the following conclusions. 1) SOUR of l/A-arch system was 2.9 Mg-O-2/9-VSS(.)min. SOUR and nitrification rate of the sludge from l/A-arch was higher than those from the l/A and A/S reactors. 2) Removal efficiencies of organic matter (TCODCr) in l/A-arch, l/A and conventional activated sludge (A/S) reactors were 93, 90 and 87%, respectively. 3) Nitrification occurred successfully in each reactor, while denitrification rate was much higher in the l/A-arch reactor. Efficiencies of TN removal in A/l-arch, UA and A/S reactors were 75, 63 and 33%, respectively. 4) Observed yield coefficients of l/A-arch, l/A and A/S reactors were 0.28, 0.41 and 0.37 gVSS/g-COD.