A method for simultaneous determination of 13 organophosphorus pesticide residues in milk samples has been developed and validated. The method is based on the extraction of the sample with acetone and petroleum ether, cleanup by gel permeation chromatography, and solid phase extraction, and determination by gas chromatography with flame photometric detection. The recovery, investigated by analysing blank milk samples spiked with azinphos, chlorfenvinphos, chlorpyrifos, chlorpyrifos-methyl, diazinon, fenitrothion, methacrifos, methidathion, parathion-methyl, pyrazophos, and triazophos at concentrations of 10, 20, and 30 µg/kg, and with parathion and pirimiphos-methyl at concentrations of 25, 50, and 75 µg/kg, ranged from 73.1% to 106.2%. Performance characteristics, such as repeatability and within-laboratory reproducibility expressed as coefficient of variation were less than 20%. Measurement uncertainty was lower than 22% for all validated organophosphorus pesticides. The limits of quantification were 10 µg/kg for all compounds and allowed determination of residues at, or even below, the maximum residue limits set by the European Union. The satisfactory z-score results of international proficiency tests confirm good accuracy, reproducibility, and reliability of the developed method.