An epidemiological study of gastrointestinal parasitosis produced by nematodes was carried out on sheep in the south of the province of Corrientes. It is important because of the importance of ovinculture, the favourable conditions for clinical presentation and the lack of systematic information. The anti-parasitic resistance environment requires foundations for the development of appropriate strategies. This work assessed the possibility of handling, based on epidemiological data and monthly surveillance. Parasitic loads in ewes and ewe lambs were monitored lengthwise over two years, kept without treatment (except wild) in a commercial mat. The assessment was carried out using the monthly necropsy of two animals in each category, reflecting the accumulated loading of ‘permanent grazing’ and two ‘tracer’ lambs which showed the infestation acquired on a monthly basis. It could be demonstrated that the higher loads accumulate in sheep in the expert during the winter, with Haemonchus contortus and Trichostrongylus colubriformis predominating, also quantitatively and qualitatively determining the profile of the overall field loads. The water balance and total water storage in the soil and its large year-on-year variation (especially in autumn) explain the parasitic loads of H. contortus, which largely determine the demand for treatment. These in turn condition the opportunity of Trichostrongylus spp. and other less represented parasites to build loads towards winter. Kernels follow a pattern of accumulation from autumn to spring, showing a more heterogeneous profile in the specific composition. Production losses are as variable as the parasitic burden, and can be presented in negligible values as this work shows. A control strategy based on epidemiological data, monitoring and monthly recording would make it possible to detect them even when they are minimal. Other factors of productive losses (mainly nutritional losses) that often overlap after parasitic disorders are highlighted. Faculty of Veterinary Sciences