Insects vector of viruses or parasites have attracted much research. Some groups of Diptera are responsible for most major diseases. This is the case of Culicidae (Diptera, Nematocera), which in many parts of the world, causing serious harm to both humans and animals through their role potential vectors of infectious diseases such as malaria, yellow fever, dengue fever and African horse sickness. Other Culicidae vector addition to their role, are a real plague through the bites painful and annoying, and thereby constitute a nuisance problem. In Algeria, in urban and suburban areas, the species most frequently encountered is that Culex pipiens, because of its wide distribution and abundance during the hot season causes unbearable nuisance. Unfortunately, attempts at chemical control against this insect often conducted in an approximate never led to the radical elimination of the target insect. Our goal is to make our contribution to try to offer an alternative to chemical control. We then initiated a toxicology study on the fourth stage larvae of Culex pipiens in the evaluation of leaf extracts of Nerium oleonder. Toxicity tests performed on the fourth stage larvae of Culex pipiens revealed after 24 hours, a sensitivity to the plant extract. The analysis of our results shows that the fourth stage larvae of Culex pipiens are susceptible to the extract of Nerium oleonder with an LD50 of 29.51 g / l. In view of its results and in the context of anti-mosquito, this extract could be proposed as natural biocides.