The effect of single-stage and 4-stage anoxic selectors and an anoxic/aerobic sequencing batch reactor (SBR) on the removal of readily biodegradable chemical oxygen demand (rbCOD) and slowly biodegradable COD (sbCOD) and on filamentous growth and sludge settleability was studied. Microbial community fingerprinting in the three selector configurations was done using automated ribosomal intergenic spacer analysis. Nostocoida limicola II and type 1851 filamentous bacteria were observed in all systems. The diluted sludge volume index (DSVI) decreased with increased selector staging. The rbCOD was almost completely removed in all selectors, and the lower DSVI values with increased selector staging were the result of greater sbCOD removal in the selector. The plug-flow kinetics in the SBR aerobic phase also improved DSVI. The bacteria community composition in the 4-stage selector system was found to be more similar to that for the SBR than for the single-stage selector.