The deposition of pesticides in the environment is affecting a wide number of living organisms which are not necessarily regarded as agricultural pests. Recently, organochlorides were replaced by less persistent organophosphorous compounds as the most used pesticides substances. Meanwhile, the environmental fate of pesticides is controlled by several processes such as retention, transformation and transportation. In addition, the environmental persistence of a given pesticide is defined by taking into account the kind and dosage of the substance being used, and their rates of biotic and abiotic degradation. On the other hand, light-induced processes are among the most important causes of pesticides degradation. These processes include direct photolysis in which a pesticide absorbs light and degrades. Another common form of degradation is the photosensitization of dissolved components in water (e.g. natural organic matter, nitrate ions or iron (III)). Moreover, climatic characteristics can play a fundamental role in both instances (e.g., temperature, solar irradiation intensity, etc.). This work presents a study on the abiotic degradation of methyl parathion (MP) from a formulated commercial compound (Folisuper- 600Br®-Agripec) in natural and ultra-pure waters under sunlight. Photodegradation experiments were prepared using ultra-pure water samples which in some cases were contaminated with humic acid (Aldrich). In addition, samples of two natural water bodies were gathered from the Zumbi dos Palmares Lake and the Paraíba do Sul River (Rio de Janeiro state, Brazil) and sterilized for biodegradation assessment. The concentration of the tested pesticide was approximately 200 μg L-1. Degradation results confirmed that sunlight is an important factor in the decomposition of methyl parathion in natural waters, especially when associated to dissolved organic materials. Moreover, when the formulation was placed in ultra-pure water samples and exposed to sunlight, MP half-life time was 11.89 days. These results are lower than those found in the system without irradiation (32 days). In addition, the half-life time was 4.89 days when humic acid was added. Under humic conditions, indirect photolysis represented an important contribution in the explanation of the degradation process of methyl parathion. Furthermore, MP showed a different behavior in natural waters. Degradation results of the lake water showed that the degradation kinetics of methyl parathion under solar irradiation is of first order and faster than in a shadow bottle. The halflife time for experiments under irradiation and in the shade were of the 4.41 and 6.89 days, respectively. Experiments conducted in the shade showed that the hydrolysis process was responsible for degrading 32% of MP in the sterilized lake water. The estimated half-life time for this sample was the highest (24 days), showing the importance of the biodegradation and photolysis processes for methyl parathion. Kinetics data from MP degradation in river water samples showed no difference when the water was sterilized. Similar degradation and t1/2 were observed in all experiments, indicating a small difference when the system is exposed to irradiation and abiotic conditions caused by compounds present in the river water. Finally, the experiments showed the formation of methyl paraoxon, a highly toxic degradation product created only when sunlight is present.