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Abstract Twohundred fish were collected as following : Sixty Tilapia nilotica and sixty Armontcatfish were collected from ponds of Faculty of Agriculture (Assiut Univeri sty ). Hiver Nile and Ibrahima Canal. Forty Tilapia mos’ampikaand forty Channel catfish were collected from ponds of International Fisheries Academy-Unitedstates of America. Fish were examinedclinically. (15.15.10.10 fish) respectively presence of clinical alternations. These fish were subjected to P.Il. parasitiologicall’, and bacteriologically examinations. The clinical alternations were a white line on the ollter margin of the fin which progresses toward the base of the fin. As the lesion develops. the ollter fin margin becomesfrayed from disintegration of the soft tissues, between the fin nqs. The degenerative process continlles llDtil the whole !1.n is lost and in the case of the caudal fin erosion. only a stump is left. Redness, haemorrhagic patches distributed allover the body and fins, Exopthalmia and anal prolapse with abdominal distension. Roughness of the scales and their detachment from some parts of the body and formation of ulcers. Parasitio10gic examination of fish revealed the presence of IcthYophthirisus mu1tifi1is protozoon infestation among the diseased fish which was considered as a stress factor. Bacteriologic examination of diseased fish revealed the pre sence 0 f nine isolates from Aeromonas 1iguefacens strains and Four isolate s from Pseudomonas• fluorescens strains which were in their morphologic cultural and biochemical behaviour closely related to the previous organisms. The biological properties of the isolates were studied by inoculation in fish. In case of Aeromonaa 1iguefacens strains seven isolate s were found viru1ent, aDd two isolates were avirulent to the tested fish. In case of Pseudomonasfluorescens strains three isolates were found virulent and to the tested fish. , one isolate is avirulent The infection of fish with Aeromonasliguefacens and Pseudomonasfluorescens were transmitted experimentally by IIp and lIM routes and not transimitted by scar~fication of the epidermis. The crowdness was essential factor for increasing the incidence of the disease amongcultivated fishes. Besides Tilapia, Catfish have been proved to be a good model for studing the biological properties of fish pathogenic bacteria |