Glugea eda n. sp. is described from the mesenteries of the striated fusilier, Caesio striata, collected from the Red Sea coast off Yanbu' al Bahr, Saudi Arabia. Numerous blackish xenomas, ranged from 3 to 5 mm, were found in the body cavity associated with the mesenteries. Mature spores are monomorphic, ellipsoidal with an average size of 5(4-6) mu m in length and 2.2 (2-3) mu m in width. Observations of the ultrastructure revealed that the development was asynchronous and that the nuclei were isolated throughout the life cycle with uninucleate meronts. Sporoblasts were uninucleated and existed together with sporonts in a fully formed parasitophorous vacuole. The polar filament of the mature spore was isofilar with 24-28 coils, arranged in three rows. Phylogenetic analysis placed the current microsporidia within the clade grouping Glugea species and close to the species described from the Red Sea and Arabian Gulf. The morphometric and molecular comparison with other members of the genus Glugea evidenced the taxonomic novelty of the present form, suggesting that it should be considered as a new species. To the best of our knowledge, the parasite here described represents the first occurrence of microsporidian infection in the fish of the family Caesionidae.