The ultrastructure of the testis of the Japanese quail (Coturnix coturnix japonica) was studied under the transmission electron microscope (TEM). The spermatogonia were characterized by spherical nuclei with little heterochromatin appearing like clumps bordering the nuclear envelope; the nucleoli were distinct. The perichromatin and interchromatin granules could be clearly distinguished. The mitochondria sometimes appeared vacuolated, with short and sparse cristae, and evenly distributed in the cytoplasm.The heterochromatin of the spermatocytes was dispersed in the nuclear sap and consisted of clumps of granules, and the nucleoli were clearly discernible. The mitochondria were few in number and similar to those of the spermatogonia in distribution. This was at variance with the distribution of the mitochondria in the spermatogonia and spermatocytes of the chicken. The rough endoplasmic reticulum and Colgi complex were well-developed and the annulate lamella could be seen in the cytoplasm.The spermatids had large and round nuclei; the little heterochromatin present appeared sp-erse and granular,being located near the nuclear envelope and scattered in the nuclear sap. The electron density of the mitochondria was rather high, and the endoplasmic reticulum and Golgi complex of the cells moderately developed.The Leydig cells of the testis were irregular in shape, with distinct nucleoli. The contained considerable quantity of endoplasmic reticulum and mitochondria, both showing polar distribution. The cristae of the mitochondria were either flat and shelf-like or tubular in shape.