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The goal of the present work was to verify the effect of Tilapia culture on net cages over the diel and vertical variation of the limnological characteristics of a reservoir from semi-arid region of Rio Grande do Norte. Samples were collected in November 2010 in the Santa Cruz reservoir at intervals of 3 hours between collections for 24 hours. Samples were collected at a site near the activities of fish production and at another location away from there. The limnological variables analyzed were: temperature, dissolved oxygen, pH and turbidity. Samples of water were collected on surface, middle (4 m) and bottom (9 m) determining values of total N, NH3, N-NO3, N-NO2, total P and chlorophyll a. It could be observed that there was no thermal stratification and chemistry of the water column during throughout the analyzed period. In general, the current activity of fish farming in net cages did not increase the values of nitrogen forms and chlorophyll a in the water column of the reservoir of Santa Cruz in the different times analyzed. However, it is important to do a long-term monitoring of limnological and biological variables and sediment in order to infer about the future impacts of fish farming activity on the water quality of the reservoir.