Isopods are permanent ectoparasite of fishes that adversely affect the fish population and so on the economic gain, therefore the present study was conducted to evaluate the prevalence and identified these parasites in some important fish species of Manzala Lake at Port-Said Governorate (Egypt) and its effects on fish quality. From February 2016 to January 2017 a total of 323 Fish sample was collected as 100 from Mugil species and 223 Tilapia species.The total prevalence was 36.22% with the highest prevalence at Spring by 43.67%; Summer 37.03% then Autumn 35.13% and the lowest incidence in Winter 28.39%.In Tilapia the total prevalence 39.46 % highest season in Spring by 46. 03% then Summer 42. 85; Autumn 37. 77% and Winter 30.50% while in Mugil spp., the total prevalence 29% The highest prevalence was in Spring 37. 5% then Summer 24% and Autumn 31. 03 and Winter 22. 72%. Two isopods spp., were isolated from Tilapia fish, Nerocila Orbiganyi and Anilorca Physodes with a total prevalence 70%and 30% respectively and one isopoda spp.,in Mugil fish Livonica
redmanii. The fish quality assessment revealed non-significant changes observed in total volatile base nitrogen (TVB-N), trimethylamine (TMA), thiobarbituric acid (TBA) and Histamine while there was a significant decrease in protein % in Tilapia fish with non-significant decrease in Mugil fish. The sensory quality and acceptability was very low for all panelists and author regard to appearance and the aesthetic presence of parasite itself onfishes