This study investigated the effects of cypermethrin, a synthetic pyrethroid, on some developmental features of Drosophila melanogaster. Before the experiment the lethal concentration of this insecticide was determined. Cypermethrin solution was applied to Drosophila melanogaster by means of nutrition, by adding it to the culture medium. In the control group no such substance was applied. Our aim was to find out whether any developmental toxic effects occur, and, if they do, during which stage, by observing the rates of egg-laying of adult females, the development of eggs, and the development of larvae. The results showed that there was no significant difference in the rates of egg-laying. However, the decrease in the rate of egg development revealed that eggs and early embryonic stages were sensitive to toxic effects. The same toxic effect was not observed in third instar larvae. The toxic effect was observed to be strongest in the early stages of development.