In this novel research study, the degradation of aqueous alachlor by ultrasonic irradiation was investigated. 575 kHz ultrasonic oxidation has obeyed first order kinetic model. The rate constant has increased with the applied power: k=2.00 x 10(-2) min(-1) (at 45W), k=2.70 x 10(-2) min(-1) (at 60W), k=13.07 x 10(-2) min(-1) (at 90W). Alachlor oxidation was achieved, leading to 100% pesticide removal for both 60 and 90W while 12% mineralization for 60W and 25% for 90W. The alachlor removal was terminated in 30 min at 90W application and the speed of the process did not allow the investigation of the intermediate and/or by-products appearing and disappearing during the pathway. For this reason the reaction mechanism was studied by using the 60W process. A detailed reaction mechanism was proposed for the degradation of 100 ppb alachlor. The operation was also evaluated by the Microtox toxicity measurement of treated solutions. The fluctuations in the toxicity levels were explained by the simultaneous detection of the pesticide and the intermediate products throughout the process. For the recalcitrant feature of the pesticide, the process required high energy ultrasonic irradiation to achieve non-toxic solutions. (C) 2014 Elsevier B.V. All rights reserved.