This study is to evaluate the utility of a strategy of stepwise increased selection pressure for cultivating aerobic granules. Three sequencing batch reactors (SBR; R1-R3) were used. Substrate ammoniui-n-nitrogen (NH4+-N) concentration was a microbial selection pressure for selecting nitrifying bacteria. It was stepwise increased from 50 to 200 mg l(-1) in R2 and fixed at 50mg l(-1) in R1 and 200mg l(-1) in R3. Results showed that aerobic granules failed to be developed in R3 due to free ammonia (FA) inhibition, while they formed quickly in both R1 and R2. Filamentous dominated granules in R1 exhibited poor performance and started to disintegrate from day 131. Stepwise increased selection pressure in R2 selected slow-growing nitrifying bacteria gradually, and this suppressed filamentous growth and improved the stability of aerobic granules. During the 283 days operation, R2 produced aerobic granules with incrementally higher biomass concentration, better settleability, and larger granule size. Larger granules further favored denitrification and the optimal granule size is suggested to be controlled within 2-3 mm. This study demonstrates the successful application of the strategy of stepwise increased selection pressure for avoiding the failure of aerobic granulation and improving the stability and performance of aerobic granules. (c) 2007 Elsevier Inc. All rights reserved.