The parasites infecting the internal organs of Oreochromis niloticus and Tilapia zillii. It also deals with the impact of these parasites either on the infected tissues or on the whole biology of the fishes under study. The specimens were collected from three different water bodies; the River Nile, El-Foadia stream and El-Mora canal in Sohag. Were examined gills, liver, gonads, spleen and intestine for the parasitic infection. This revealed the presence of eight species of genus Myxobolus, One species related to genus Sphaerospora was described new species, spores related to genus Trichodina were detected in gills, Monogenean and Digenia worms were isolated from Oreochromis niloticus and Tilapia zillii. The impact of parasites was clear on the various tissues and led to the tissue damage and destruction of some of the cell. vary the intensity of the spread of the parasite on the environment in which it resides Fish. There is a clear relationship between the length of the fish and density of parasites in fish.Increased parasite density inside the ovary impacted on fecundity in Fish.