A carbon paste electrode is modified with ZrO2-nanoparticles and its application for square wave voltammetric (SWV) detection of methyl parathion, MP (organophosphate pesticide) is reported. The nano-ZrO2 shows a strong affinity toward the phosphate group on methyl parathion molecules, which provides sensitivity and selectivity of the modified carbon paste electrode. Instrumental and chemical parameters influencing the voltammetric response are optimized for MP determination. Under optimum experimental conditions the peak current is linear to MP concentration over the range of 5.0-3000.0 ng mL(-1) with a correlation coefficient of 0.9992. The limit of detection (LOD) and limit of quantification (LOQ) for MP based on three and ten times the standard deviation of the blank (3S(b), 10S(b)) were 2.0 and 5.7 ng mL(-1) (n = 20) for MP, respectively. The recovery values from quality control (QC) samples of water solutions containing low, middle and high concentrations of MP (50, 100 and 800 ng mL(-1)) were 98.0 +/- 2.3%, 92.5 +/- 4.9% and 97.6 +/- 2.8%, respectively. The electrode is successfully applied for the determination of MP in different water samples. (C) 2010 Elsevier BM. All rights reserved.