In the present study, a haplosporidian parasite was detected in the freshwater mussel Caelatura aegyptiaca. This mussel was collected from a site located on the eastern bank of the River Nile about 21Km north of Sohag city. The haplosporidian parasite was found to inhabit different organs of the host such as gills, mantle and foot. Their epithelial and connective tissues were filled with different stages of haplosporidian parasite. These stages are uni- and binucleated trophozoites, plasmodial stage, sporoblasts and immature and mature spores. Histopathological sections showed that the epithelial and connective tissues of host infected organs are hypertrophied, stretched with acute swelling, epithelial fusion, liquefactive necrosis and network fragmentations. In addition, epithelial cells became dense and have darkly stained nuclei and vacuolated cytoplasm. The infected organs showed haemocytic infiltration as an internal defense reaction of the host.