Microorganisms compete with other species by secreting antimicrobial compounds. The compact structure of aerobic granules was generally assumed to provide spatial isolation, resulting in the co-occurrence of diverse strains that have similar or dissimilar functions. No studies have investigated whether stable, mature aerobic granules can be formed with two mutually inhibitory strains. The strain Acinetobacter sp. 18 competes with Bacillus sphaericus 15 in a well-mixed environment, but can form stable and mature granules at 400 mg L-1 phenol by repeatedly replenishing fresh medium in a sequencing batch reactor. The supernatants collected from the 18 medium in its exponential-growth phase or from the 15+18 medium cultivated for 12 or 24 h significantly inhibited 15 growth. Addition of tightly bound extracellular polymeric substances (TBEPS) or loosely bound extracellular polymeric substances (LBEPS) extracted from 15+18 granules effectively suppressed the inhibitory effects of 18 on 15. The TBEPS or LBEPS physically separate strain 15 from 18 in the granule, and effectively adsorb the inhibitory substance(s) in the suspension. (C) 2009 Elsevier B.V. All rights reserved.