Öz Thiotepa was applied at two doses (0.01 % and 0.1 %) and for 3 or 10 minutes to the abdomens of A. ipsilon larvae. The coating layers of ovaries are distinctly affected by 0.1 % Thiotepa for each period. The cells in the ovarioles shovv, also, degenerations after using 0.01 % and 0.1 % Thiotepa. 0.01 % Thiotepa application for three minutes has a less effect. The increase in concentra- tion as iveli as the duration of application of Thiotepa increases also the degeneration more severely in the cells of ovarioles. A complete destruction is obtained after the application of 0.01 % Thiotepa for 10 minutes. The apperances of these degenerations are shown ıvith electronraicrographs in detail Thiotepa is a vcry effective chemosterialant for A. ipsilon.