AbstractA new myxosporean,Myxobolus bulbocordissp. nov., has been found in the heart of a Mesopotamian fish,Barbus sharpeyi(Günther), in rivers in Southwest Iran. Cysts containing matured spores were located on the serosa of the atrium cordis, bulbus arteriosus, larger gill arteries and inside the wall of the bulbus. The species showed an affinity to connective tissue cells and was never associated with the muscles. Mature cysts were surrounded by a connective tissue capsule composed of two to three layers. Spores in disrupted cysts were infiltrated by epithelioid cells and macrophages. The spores found inBarbus sharpeyidiffered in size and morphology from species known from other barbels