This study investigated the effects of diazinon, an organic phosphoric insecticide, on some developmental features of Drosophila melanogaster. The aim was to determine if any developmental inhibition occurs when diazinon is applied and if so, to determine which developmental stages are affected. Therefore, diazinon was applied to Drosophila melanogaster eggs, larvae, and adults, and its effects were observed. During the experiments 3 concentrations with values lower than LC were utilized (6 x 10(-5), 6 x 10(-6), 6 x 10(-7) ppm). Diazinon solution was applied to Drosophila melanogaster by means of nutrition, adding it to the culture medium. The results show that development from egg to third instar larva was significantly inhibited in the F, and F, generations. The development of larva to adult was only inhibited in the F, generation. The application of diazinon to females did not cause any change in the rate of egg laying. The overall results show that diazinon's toxicity was strongest during the early stages of development; however, it also affected the F, generation.