Parasitological research today focused on studying protozoan parasites infecting fish especially myxosporidian and microsporidian parasites which cause adverse effects on the fish health causing high losses in fish production. Also the presence of Heavy infections with muscle-invading species destroy the texture of the fish flesh making it unmarketable. Morphological description of these parasites help in the exact determination of their taxonomy which may aid with the molecular tools in the development of anti substances against these parasites. The present book described some of these parasites including two myxosporidian and three microsporidian species infecting flesh of four of the most economically important marine fishes of the Red Sea in Egypt. The study described the different developmental stages of these parasites which undergoes its maturation within fish flesh until its progressive development into the infective spores which are the most dangerous and infective part of the cycle.