Adult male Swiss albino mice Mus musculus were administered three different oral doses, such as, 33, 66 and 99 ppm of endosulfan per day for 4 weeks. Treated and control specimen of mice were sacrificed on 14th and 28th day, and changes in their testicular cells were observed. Light microscopy study demonstrates that the changes occurred mostly in seminiferous epithelium, intertubular space, spermatogonia, spermatocytes, spermatids and spermatozoa. Major changes attributed to the testis after endosulfan treatment in Mus musculus are fusion, invagination, distortion and disintegration of seminiferous tubules, autolysis of cells, arrest of spermatid formation and of spermatogenesis, loss of spermatozoan tail and decrease in number of interstitial cells. Other such abnormalities and the degree of impairment in the testicular cells increased with the increase in concentration of endosulfan.