The　morphological　and　structural　characteristics　of　aerobic　granulation　were　investigated　in　a　sequencing　batch　reactor.　Small　granules　consisting　mainly　of　bacterial　microcolonies　first　formed　at　an　organic　loading　rate　of　1.68　kg　chemical　oxygen　demand　(COD)　m(-3)　d(-1)　and　a　minimal　settling　velocity　of　9.6　m　h(-1).　Mycelial　granules　consisting　of　fungi　and　filamentous　microorganisms　formed　as　the　minimal　settling　velocity　increased　to　18　m　h(-1).　Scanning　electronic　microscopic　observations　indicated　that　fungi　formed　the　structural　backbone　upon　which　other　microorganisms　can　attach　and　colonize.　The　attached　organisms　began　to　reproduce　themselves　and　quickly　fill　up　the　spatial　voids　within　the　granules.　When　the　organic　loading　rate　of　the　reactor　increased　to　4.2　kg　COD　m(-3)　d(-1),　the　granules　exhibited　a　denser　and　more　compact　structure　and　bacteria　with　a　variety　of　morphotypes,　embedded　in　extracellular　polymeric　substances,　dominated.　When　the　aerobic　filling　time　was　extended　from　5　to　30　min,　the　thick　and　compact　granules　were　gradually　shifted　into　a　light　and　loose　filamentous　granular　structure.　The　extension　of　the　aerobic　filling　time　eventually　led　to　instability　and　the　failure　of　reactor　operation.　(c)　2006　Society　of　Chemical　Industry.