Marine cultured ridgetail white prawn, Exopalaemon carinicauda, in Qidong, Jiangsu province showed a typical sign of muscle whitening. The muscle in abdomen, cephalothorax, pereiopoda and tail fan all exhibited clearly pathological changes. Microscopical examination of fresh muscle demonstrated that there were lots of spherical cysts in the tissue. Further, morphological and structural analysis confirmed that the pathogen belonged to the species of microsporidian by using ultramicroscopic technique. Specifically, the pathogen was characterized by sporophorous vesicle structure which usually contained eight spores and was (5.4 ± 0.55) μm in size. The mature spores were oval shaped, approximately (2.3±0.25) μm in length and (1.5±0.19) μm in breadth. The polar filament was wrapped around the inside of the spore into 9-10 coils. In addition, pathological analysis showed that the parasite invaded the gill, stomach, gut and muscle tissues of host and caused pathological changes in different degree. Among these tissues, the skeletal muscle was damaged most severely. Muscle fibres in the abdomen were largely replaced by spherical sporophorous vesicles. Myofibrils were ruptured apparently. The number of vacuoles increased and the mitochondrion disappeared. The gill tissue exhibited signs of hypertrophy, vacuolization and membrane necrosis with the disappearing of nucleoplasm, cytoplasm and organelles. Electron density of cytoplasmic matrix in the stomach cells increased obviously and spores mainly distributed around the lipid droplets in the connective tissue. Whereas in gut, the parasite mainly distributed between the submucosa and muscle layer with the nucleoplasm of epithelial cells condensed and the muscle layer being destructed.