The effects of anoxic and oxic fill strategies on SBR performance under nitrogen (NH4Cl as the nitrogen source) deficiency and rich conditions are evaluated using glucose as the sole substrate. The performance is evaluated according to substrate removal, sludge settleability, supernatant suspended solids (SS), and reactor biomass concentration. Substrate removal efficiencies are found to be independent of the fill strategies adopted under all conditions tested. The incorporation of anoxic selector environment fails to prevent the development of bulking sludge under nitrogen deficiency conditions thereby resulting in a gradual depletion of reactor biomass. Under nitrogen rich conditions, on the other hand, the sludge settleability improves significantly by adopting the anoxic fill strategy. Furthermore, SS readings taken at the end of settling period are greater with anoxic fill than with oxic fill, indicating that the latter discourages the growth of dispersed bacteria.