One hundred of each Oreochromis niloticus, Sarotheroden aureus and hybrid tilapia (all male), 25 fish / season, were collected from Fish Research Center, Seuz Canal University during the period from March 1995 through February 1996. The fishes were examined for clinical signs, postmortem lesions and skin ectoparasites. A comparative studies were done for the skin ectoparsitic affection among the three species of tilapia along with their prevalence, seasonal occurance and the histpathological changes. Besides, the control of detected skin ectoparasitic diseases among tilapia species in the reared ponds was performed using the static procedure by formalin. The skin ectoparasitic prevalence among O. niloticus, S. aureus and hybride tilapia was 53, 51 and 84% respectivelly while the clinically diseased fishes were 9, 8 and 20% respectivelly. The prevalence of different skin ectoparsites among 0. niloticus, S. aureus and hybrid tilapia were 28, 26 and 50% for protozoa including Trichodina fultoni, Chilodonella hexastica and Myxobolus dermatobia ; 10, 11 and 14 for trematodes including Gyrodactylus elegans and encysted metacercaria (Postaodiplostomum cuticula); 3, 2 and 5% for crustacea (Lamproglina mondi); and 12, 12 and 15% for mixed infection of protozoa and trematodes respectivelly. The clinical signs and P.M. lesions of skin ectoparasitic diseases were restlessness, rubbing the body against hard objects, excessive mucous secretion, roughness of the skin with scale loss, petichial hemorrhages and focal to large hemorrhagic ulcers. In casesinfected with encysted metacercaria, the lesions were skin black spots. Histopathologically, vacuolar degeneration and necrosis in the epidermis were commonly observed in most infested cases. Formalin at concentration of 1 : 4000 (250 mg/L) for one hour in static procedare succeded to control the detected skin parasitic diseases among different tilapia species indicated by absence of the skin ectoparsites and gradual disappearance of the clinical signs.