The present study aimed to isolate and identify of Aphanomyces invadans from naturally infected African Catfish" Clarias gariepinus". A total number of 105 cultured Clarias gariepinus collected from private fish farms in Kafr Al-Sheikh & Behera Governorates. Result revealed that the prevalence of infection among the examined fish was 8.6 % and the disease mostly recorded during winter and spring. The infected fish developed characteristic ulcerative lesions and fin rot. Fungus-like oomycetes

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... anomyces invadans grow on glucose peptone yeast (GPY) agar as opaque colonies with uneven white transparent velvets surface at room temperature. Microscopically, lactophenol cotton blue stained fungal growth appeared as non-septated thin long branched hyphae with tapered end containing cytoplasmic organelles. On sporulating media, the rectangular shaped spores appeared inside the hyphae connected together by thin filament. In conclusions, EUS is an invasive disease of Clarias gariepinus and care should be taken with low temperature in managing fish pond.