This study was conducted to investigate the fungal biota of cultured O. niloticus from Abbassa Fish Farms. Fishes were subjected to clinical, postmortem, mycological and histopathological examinations. The obtained results showed that, (Saprolegnia spp. Cladosporium sp., Chrysosporium sp., Scopulariopsis brevicaulis, Aspergillus versicolor sp., Aspergillus ochraceus, Aspergillus niger, Aspergillus glaucus, Aspergillus wentii, Rhodotorulla sp., Penicillium sp., Syncephalastrum sp., Aspergillus tamarii., Aspergillus flavus, Trichoderma sp., Acremonium sp., Rhizopus sp., Fusarium sp., Phoma sp., Absidia sp., Aspergillus candidus, Alternaria sp., Aspergillus terreus, Aspergillus  fumigatus and Paecilomyces sp.) were isolated from the examined fish. Penicillium sp. is the most fungus distributed all over the year. Clinical signs and postmortem lesions of examined fish were slow swimming, loss of appetite, scale loss, ulceration, severe erosion and hemorrhagic skin, with septicemic picture in internal organs. Most histopathological alterations of infected fishes were recorded in skin, gills, liver and kidney and discussed.