Abstract: This study completely descripted Protoopalina limnocharis Nie 1932 at both light and scanning electron microscope levels. These parasites were collected from the frogs Fejervarya limnocharis from Honghu Lake, Hubei Province, China in July 2016. The body of P. limnocharis was elongated and somewhat ribbon-like in shape, with its length/width ratio about 10﹕1. The widest position of the body was at a region slightly anterior to the middle, while it tapers at both ends. The body surface was thickly flagellated. All the somatic kineties converge on the falx, anteriorly, and many extend to the posterior extremity and number 30﹕40 in total. The falx, composed of a narrow band of kinetosomes, observed in protargol-impregnated specimens at a higher magnification that located at the margin of the anterior pole and extends to both dorsal and ventral sides. Two oval-shaped nuclei observed inP. limnocharis, one of which is situated at the anterior while the other is in the middle. The two nuclei were connected by a long thin thread, which is formed in the process of nuclear division and kept during all the life history of the opalinids. These two aforementioned characteristics were compared between P. limnocharis and its congeners, such as P. caudata michyla, P. heleophrynes, P. pomacantha and P. quadrinucleata. The feature of falx is constant to the opalinids’ taxonomy. Besides, the thin thread between two nuclei of P. limnocharis is the remaining trace during the evolutionary history of nuclear division.