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The present study deals with the infestation study of Livoneca redmanii (Isopoda, Cymothoidae) on Mugil cephalus in Lake Qarun, Egypt. Out of 576 examined fish collected monthly from the different localities of Lake Qarun during the period (January - December, 2016) there are 269 (46.7 %) fish were infested by crustacean parasite. Results showed that the highest infestation percentage in Mugil cephalus was 76.20 % recorded in February and the lowest one occurred in January (19.40 %). Generally, the infestation percentage in male fishes is lower than those in females being, 34.6 % and 65.4 % respectively. In case of males the highest infestation percentage occurred in July being, 65.5 %, while in case of females the highest percentage was 72.1 % in March. The isopod parasite, Livoneca redmanii, preferred medium size fish (14-17 cm) with infestation percentage 70.64 % and fishes that had weight ranged between 12 g and 14 g with infestation percentage being, 78.82 %. Also, this parasitic isopod infected gill bilateral and/or unilateral infection per hosted fish with percentage being 98.1 %. The main clinical signs in infected fishes appeared in slow swimming at the water surface, extensive mucus secretion, increased opercula movement and some fishes aggregated on the surface and accumulated at the fresh water inlet. Examination of post mortem lesions revealed that presence of ulcer and erosion blew the gills at the site of parasite and skin abnormalities such as skin ulcers, scale-less and discoloration. Histopathological examination of infected gills showed hyperplasia of epithelial cells in gill filament. Hyperplasia of lamellar epithelium and mucous secreting cells tend to be confusion between secondary gill lamellae, lifting of secondary lamellae and congested blood vessels were also noticed. It is now clear that cymothoid isopod, Livoneca redmanii was causes economic loss associated with reduced fish growth, high mortality and marked drop in fish production from Lake Qarun. [ABSTRACT FROM AUTHOR]