Aphanomycosis was reported in about 8% of the examined Clarias gariepinus. The disease was recorded during spring and autumn. Aphanomyces fungus was found as sparsely branched-thin-long non septated hyphae in squash preparation from autolysed infected tissues. The organism was isolated on glucose peptone yeast broth using 5 steps culture technique with some modifications as addition of metalexyle, amphotricin B and fluconazol (antimycotics). Culture characters on glucose peptone yeast broth and agar were studied. On sporulating media, sporangia were formed at the hyphal tips where spores encysted in clusters from which secondary zoospores released. Biochemical identification of the isolated fungus was recorded. Clinical signs and lesions on naturally and experimentally infected fish were also described. Histopathological examinations revealed presence of severe inflammatory reaction with heavy inflammatory cellular infiltration and multiple granulomas formed from aggregation of inflammatory cells mainly epitheliod cells. Muscle degeneration and necrosis were observed. Numerous multinucleated gaint cells were found and some of them showing fragmented fungal parts within their cytoplasm. Moreover, PAS positive spores and fungal hyphae were noticed.