Agricultural pesticides play a significant role in Algeria's agricultural sector, helping to protect crops from pests and diseases, and thus contributing to food security and agricultural productivity. The objective of this study is to test the insecticidal activity of Oxam (Thiamethoxam 25%), on Culex pipiens (Linnaeus, 1758) mosquito. For this, various concentrations (5, 10, 20 and 50 µg/l) were tested on fourth-instar larvae newly molted from the domestic mosquito Culex pipiens. Three repetitions for each concentration were carried out with 15 larvae each. Preliminary trials allowed for the selection of this range of concentrations. After estimating LC50 and LC90, they were applied to newly laid egg rafts to determine their ovicidal activity. The results obtained showed that the used pesticide has a larvicidal effect on fourth-instar larvae, as evidenced by the mortality recorded in relation to concentration compared to the control. The lethal concentrations LC50 and LC90 were estimated at 9.54 and 22.9 µg/l respectively. Additionally, it exhibits ovicidal activity by reducing the hatching rate from 96% in the control group to 93% and 85% for LC50 and LC90 respectively.