In the present study, attempts are mode to detect the parasitic tissue encystations (Haplorichid and Echinotsomatid encysted metacercaria, myxosporean cysts and non parasitic tissue encystations Epitheliocystitis, “chlamydial infections of fish “). In gills of Oreochromis niloticus fish. The clinical signs of naturally infected freshwater cultured Oreochromis niloticus are recorded. The prevalence of tissue encystations (encysted metacercariae, myxosporean cysts and Epitheliocystis) was 80%, 46.6% and 69.3%, respectively. The isolated parasites identified as Haplorichid, Echinostomatid encysted metacercaria and 2 types of reported in the present work for the first time from Egyptian fresh water fish. Light microscope detect intracellular granular inclusion of Epithhelocystis, while electron microscopic examination revealed that the pathogen are rod shaped and situates at the center and had an electron dense core with electron lucent vesicles on its sites.