Myxobolus Bütschli, 1882 (Myxozoa, Myxosporea, Bivalvulida, Myxobolidae) is an important parasitic protozoan of freshwater fishes reported from almost the worldwide. The severity of infection may lead to mortality of the host fish. During a parasitological survey on parasites infecting fishes caught from Euphrates River at Ramadi City, Iraq during regular visits between March and September 2016, 8/26 specimens of binni fish, Barbus shapreyi (F: Cyprinidae)
were found to be naturally infected with Myxobolus caudatus. The infection was recorded as small whitish plasmodia embedded in the caudal fin of infected host. The number of plasmodia per infected fish was 3-4 and measured 0.25-0.46 (0.37±0.02) mm in diameter. After rupture of plasmodia, the liberated spores were elliptical in shape, measured 11.6-13.7 (12.9± 2) μm in length and 6.4.0-8.3 (7.4±2) μm in width. The polar capsules were elliptical, equal in size and pointed anteriorly. They measured 7.2-8.5 (7.5±0.5) μm in length and 2.2-4.3 (3.4±0.5) μm in width. Examination of ultrathin sections through the parasite plasmodia by transmission electron microscope showed that the process of capsulogeneis followed the usual pattern of bivalvulid spore development with asynchronous type of polar capsule maturation. Also, the formation of polar filament (6-9 coils) occurred after the fusion of external tubules with capsular
primordia. The process of valvogenesis also described when two valvogenic cells met at one pole of the spore with a sutural line in between. By comparison between the isolated spores in the present study and those isolated previously from fish hosts at Iraq water sources, it showed the present parasite to be complete different with new host and locality records in Iraq.