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• Ultrasonic treatment effectively enhanced the degradation of parathion methyl (PM). • The degradation of PM was properly fitted the first order kinetics. • Three degradation products of PM were determined by UPLC-QTOF/MS. • The toxicities of three degradation products of PM were evaluated. • Among the degradation products, two of them were more toxic than PM. The removal of pesticide residues in food by ultrasound has attracted more attention in recent years, and the formation of intermediate products may have some profound effects on the toxicity of treated food. Therefore, degradation of parathion methyl (PM) in bovine milk by ultrasonic treatment was studied in this paper. Results showed that the ultrasonic intensity and the initial concentration of PM had a significant effect on the degradation rate of PM (P < 0.05). The maximum degradation rate of PM was 97.10%. Three transformation products were identified through UPLC-QTOF/MS analysis, and the oxidation pathway was proposed as the consequence of ultrasonication. Furthermore, according to Quantitative Structure Activity Relationship (QSAR) model prediction, the ecotoxicity of the transformation products may be higher than that of PM. These findings showed that although ultrasonic treatment can effectively degrade pesticide residues in food, it may also generate transformation products with the higher ecotoxicity. [ABSTRACT FROM AUTHOR]