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This article discusses the degradation of methyl parathion (MP) in natural and sterilized waters. Experiments were prepared using natural waters gathered in two aquatic systems (Rio de Janeiro State, Brazil), ultra-pure water and humic water solution under different conditions (i.e. in the presence/absence of light, sterilized/no sterilize solutions). The exposition to sunlight was carried out using experimental bottles without headspace immersed in a swimming pool for temperature control. Natural waters results showed that the degradation kinetic of MP is of first order and the half-lives for lake water experiments, under direct sunlight and shade, were 4.41 and 6.89 days, respectively. The kinetic curve for MP degradation in river waters showed that there are no differences when samples were sterilized and placed (or not) under shade conditions, and the half-lives ranged from 5.37 to 2.75 days for sterilized river water/absence of sunlight and natural/presence of sunlight, respectively. Therefore, our results showed that photolysis plays, in addition to bio- and chemical degradation, an important role in the decomposition of MP in aquatic environments. [ABSTRACT FROM AUTHOR]