The symptom of muscle whitish occurred in marine cultured ridgetail white prawn, Exopalaemon carinicauda, in Qidong, Jiangsu province. The muscle of abdomen, cephalothorax, pereiopoda, tail fan showed clearly pathological changes. The microscopical examination with wet mount of vivo revealed many spherical cysts existed in muscle tissues. Morphological and structural analysis of pathogen confirmed it was the microsporidian by using ultramicroscopic technique. The pathological analyses of infected tissues were also performed. The results indicated that this microsporidian was featured by sporophorous vesicle structure. Sporophorous vesicles containing eight spores measured 5.4 ± 0.55 (4.4~6.6) μm. The mature spores were oval-shaped, approximately 2.3±0.25 (1.7~2.8) μm ×1.5±0.19 (1.2~1.9) μm with 9-10 polar ?lament coils. Pathological analysis showed that the parasite invaded the gill, stomach, gut and muscle tissues of host and caused different pathological changes, especially worst in the skeletal muscle. Muscle fibres of the abdomen were largely replaced by spherical sporophorous vesicles. Myofibres were ruptured apparently. The number of vacuoles increased and the mitochondria disappeared. The hypertrophy and vacuolization, part of gill membranes necrosis, disappearing of nucleoplasm, cytoplasm and organelles were observed in gill cells. The electron densities of cytoplasmic matrix in the stomach cells increased and the spores lived around the lipid droplets in connective tissue. The nucleoplasm of the gut epithelial cell condensed and some muscularis disappeared. Parasites located between the submucosa and muscle layer in gut.