The effect of sludge retention time on biological nutrient removal in a sequencing batch reactor was investigated in this study. The effect of three different sludge ages (10, 15 and 25 d) on phosphorus release and uptake as well as on nitrification and denitrification was tested in a lab scale sequencing batch reactor having a simultaneous fill and draw system operating according to anaerobic-anoxic-aerobic principles. It was found that the system possessed better P removal (98 %) with shorter SRT (10 d). The phosphorus concentration in the effluent changed between 0 and 1.0 mg/l at this sludge age. When SRT was increased to 25 d, in contrast to decrease in effluent ammonium concentration (3 mg/l), P concentration increased in the effluent (5 mg/l). The system showed a very stable chemical oxygen demand removal efficiency during the entire operational period.