It was studied a process of a sexual reproduction in homothallic alga Closterium navicula, Conjugation in C. navicula (Brebisson) Lutkemuller results in production of single zygospore. Pairing symmetrical cells occurs prior to papillae formation. Papilla from one gametangial cell frequently rises out faster and larger than the other one. Previous to fusion of gametic protoplasms, papillae fuse to form one broad canal within the gametic protoplasm fuse and form one zygospore. The zygospore is slightly rounded with smooth wall.