In this study, the effects of diazinon on morphology and sex ratio in adult individuals of Drosophila melanogaster were investigated. Diazinon solution was applied to Drosophila melanogaster by means of nutrition. In the F1 and F2generation, quite a lot phenotypic abnormalities were seen. Therefore, the inheritance of the mutations caused by diazinon were investigated. For this purpose, individuals with normal and abnormal phenotypes, which developed in culture medium containing diazinon, were crossed and a second F2 generation was obtained. High level of phenotypic abnormality was found in this generation, too. Based on the data obtained as a result of this two-way test, it can be concluded that diazinon causes hereditary mutations. Although there were some differences in the data about sex ratio that arose due to the effect of diazinon, this situation did not appear to be statistically meaningful. However, female individuals are more sensitive to diazinon