Abstract: The present paper is a report on the effect of Thio-TEPA on the oogenesis of the common housefly,Musca domestica vicina.Histo-pathological changes chiefly in the germarium,and only one egg chamber was formed in each ovariole.Follicular cells in the egg chamber were severely disranged and chromatin condensation could be frequently observed,followed by pycnotic changes in the nucleus and vacuolation of the cytqplasm.Similar changes occurred in nutritive cells,but to a lesser extent.The oocyte was least affected,the only pathological change was reduction of size and failure to develop beyond 6th stage.This was possibly due to lack of supply of nutritive substances from the follicular and nutritive cells.It seems therefore that cells in active mitotic division were most easily affected.The mode of action of Thio-TEPA in causing sterility in the female housefly was discussed in relation to these pathological changes.