In this work, an automatic strategy for acclimation of biomass to toxic wastewater biodegradation in a sequencing batch reactor (SBR) is proposed and evaluated. It is based on monitoring the airflow needs for regulating the dissolved oxygen concentration at a desired set point. The regulation is achieved by using a PID controller with an input/output linearizing feedforward part, using the airflow, rate as input variable. A simple algorithm to detect the end of the reaction phase using this manipulated signal is proposed. The strategy Was tested on a laboratory bioreactor using 4-chlorophenol as model toxic compound and two inocula from different sources. The results show that it is a viable alternative for automatic acclimation of biomass for toxic wastewater treatment in a SBR. Furthermore, the strategy is also suitable for the operation of the reactor after acclimation.