ABSTRACThis study was carried out on 476 wild marine Dusky Grouper (Epinephelus guaza), captured from the West North shore of Mediterranean Sea at Matrouh Governorate from Sept. 2005 to Sept. 2006. The total prevalence of microsporidia among wild Epinephelus guaza fish was found to be 301 (63.23 %). The investigated fish, revealed the presence of cyst-like nodules ( xenomas, 2-4 mm. in diameter) in the gill arches. Each gill arch was infected with (1-5) xenomas and up to 30 nodules per fish. The recovered spores of Glugea anomala appeared as thick walled, uni-nucleated, pear-oval in shape with large vacuole. They measured (3-4 µm) in width and (5-6 µm) in length. The highest prevalence (82.9%) was recorded in summer, while the lowest one (30.33%) was recorded in winter. Presently, the highest infection rate (88.8%) with Glugea anomala was recorded among fish with 200-250g body weight, while the lowest one (43.3%) was recorded among fish of 100-150g body weight. The main histopathological alteration in gills was destruction of gill filaments with degenerative changes and infiltration of mononuclear leukocytes. The water parameters revealed the highest pH (7.3), chloride value (240 mg/L) and the total hardness (290 mg/L) in summer, while the lowest pH (5.7) and the total hardness (217mg/L) were recorded in winter, but the lowest chloride value (122 mg/L) was recorded in autumn.