A submerged membrane bioreactor (SMBR) treating ammonia-bearing synthetic inorganic wastewater without sludge purge was studied in respect to nitrification and microbial parameters over a period of 210 days. The reactor was operated at a hydraulic retention time (HRT) of 24 h, and the NH4+-N volumetric loading rate increased from 0.18 to 1.30 kg NH4+-N m(-3) d(-1) by increasing influent NH4+-N from 180 mg/l(-1) to 1300 mg/l(-1). With the exception of a short period after the failure of pH control, the NH4+-N+ removal rate was constantly above 99%. Due to the influent characteristics and the interception of the membrane module, the mixed liquor suspended solids (MLSS) varied from 3000 to 5000 mg/l(-1) and the nitrifying bacteria were dominant in the SMBR. The numbers of ammonia oxidizers and nitrite oxidizers in the mixed liquor increased from 0.9x10(8) ml(-1) and 1.x10(8) ml(-1) to 1.6x10(9) ml(-1) and 9x10(8) ml(-1), respectively, and the specific nitrification rate from 0.27 to 0.56 g NH4+-N g(-1) SS-1 d(-1). An increasing occurrence of extracellular polymeric substances (EPS) around microbial clusters with operation time was clearly observed on scanning electron micrographs (SEM). The pressure difference of the membrane module was not over 0.01 mPa over the whole operation period. Present results show that SMBR can be operated efficiently and stably as a high-rate nitrifying technology.