The theoretical basis, practical implementation, and experimental validation of an on-off control strategy for dissolved oxygen (DO) in aerobic reactors of pilot-scale activated-sludge plants with simultaneous estimation of oxygen uptake rate are presented. The effect of dead time and a long response time for robust DO probes has been carefully analyzed and corrected through modeling of the dynamic response of the sensor. Study results indicate that the amplitude of the control band depends on the dead time of the probe. Simultaneous estimation of the volumetric mass-transfer coefficient for DO (KLa) permits a continuous check of the aeration equipment. Experimental validation has been achieved in a continuous pilot-scale plant.