Abstract: In this paper, the primordial germ cells (PGCs) and gonad differentiation of Clarias lazera are studied by means of light-and electron-microscope. The germ plasm associated with the germ cells is found in the cytoplasm of the PGCs and developing oogonia and spermatogonia. The PGCs come from the endoderm which is near the periblast nucleus. The PGCs have the ability to move actively during migration. After reaching the site of the future gonad, the PGCs and epithelial cells form the primordial gonad which gradually differentiates into testis or ovary. The germ plasm is closely associated with the differentiation of the primordial gonad. The ovary appears earlier than the testis.