Some fishermen use contaminated food in the bottom traps and barriers in the Nile branches to attract fish. Microbiological investigations showed that such food is highly contaminated by bacteria and fungi. The identified genera of bacteria in food material, water and skin lesions were Aeromonas, Pseudomonas* Bacillus and Actinomycetes. On the other hand, four species of fungi namely Aspergilhts flavus, Aspergillus lerreus, Fusarium sp. and Penicillium chrysogenum were isolated from the contaminated food. The field observations revealed that Tilapia zillii samples collected from the area contaminated by such food had higher ratio of skin diseases compared to other sites. The skin diseases in this site included skin ulcer (13.8%), fin erosion (16.9%) as well as b -u. ions (10%). The ratios of the skin ulcers (>15%) and fin erosion (>30%) of the total body area among male and female T. zillii from the contaminated site were elevated compared to other ratios. The seasonal variation of different skin diseases revealed that the maximum prevalence, ranging between 40.8% and 24.2%, were detected in summer and spring respectively. The GLM statistical analysis revealed that site and season had significant effects (p15%) versus such biological variables showed that feeding intensity (FI), fish condition (k) and hepato-somatic index (HIS) of males decreased with increasing the area of skin ulcers, while TSI was not affected by this lesion. The trend was more obvious in case of females, where FI5 K, HIS and OS1 were negatively affected by the skin ulcers. On the other hand, FI and K of males and FI, K and OSI of females were inversely correlated with severe fin erosions (>30%). Laboratory investigations showed that the skin ulcers and fin erosions first appeared in the 5th day after the application of the contaminated food with or without water from site (B) in the aquaria. The mortality rate in fish, which were provided with the contaminated food with or without water from site (B), was 10 % at day'18th and 33.3 % at days 19th and 20th, reaching 100 % at 22nd day.