Two sequencing batch reactors (SBRs) were operated for 100 days under aerobic conditions,with one being fed with unsterilized municipal wastewater (USBR),and the other fed with sterilized municipal wastewater (SSBR).Respirometric assays and fluorescence in situ hybridization (FISH) results show that active nitrifiers were present in the unsterilized influent municipal wastewater.The maximum ammonia utilization rate (AUR) and nitrite utilization rate (NUR) of the unsterilized influent were 0.32 + 0.12 mg NH4+-N/(L.hr) and 0.71 ± 0.18 mg NO2--N/(L.hr).Based on the maximum utilization rates,the estimated seeding intensity for the ammonia oxidizing bacteria (AOB) and nitrite oxidizing bacteria (NOB) of the USBR was 0.08 g AOB/(g AOB.day) and 0.20 g NOB/(g NOB.day) respectively.The fraction of nitrifiers/total bacteria in the influent was 5.35％ ± 2.1％,the dominant AOB was Nitrosomonas spp.,Nitrosococcus mobilis hybridizated with Nsm156,and the dominant NOB was Nitrospira hybridizated with Ntspa662.The influent nitrifiers potentially seeded the activated sludge of the bioreactor and hence demonstrated a mitigation of the acclimatization times and instability during start-up and early operation.The AUR and NUR in the USBR was 15％ and 13％ higher than the SSBR respectively during the stable stage,FISH results showed that nitrifiers population especially the Nitrospira in the USBR was higher than that in the SSBR.These results indicate that the natural continuous immigration of nitrifiers from municipal influent streams may have some repercussions on the modeling and design of bioreactors.