Thaumetopoea pityocampa larvae are very harmful to pines and they also cause allergic reactions in men and animals. In this study, different concentrations of endosulfan were administered to T. pityocampa larvae via pine needles which were prepared by the dipping method. The data obtained were statistically evaluated using probit analysis and a LC50/48hrs value for T pityocampa larvae found to be 1.679 mg/l. Also, 12,24, 36 and 48 hrs after 1.679 mg/l endosulfan treatment, ultrastructural changes in the midgut epithelium of T. pityocampa were investigated. No pathological changes were observed after 12 hrs, swelling and vacuolization of mitochondria and dilation of endoplasmic reticulum after 24 hrs, swelling of mitochondria and breaking of mitochondrial cristae and dissolving of nucleoplasm after 36 hrs, finally large vacuoles in the midgut epithelium cells were observed after 48 hrs.