During last spring (2006) severe mortalities among cultured monosex tilapia were recorded in a private fish farm at Alexandria governorate and caused up to 20% mortality. The isolated bacteria was identified as Streptococcus faecelis. Experimental infection was successful through intraperitoneal injection (i.p.). The clinical signs were sluggish movement, respiratory disorder, darkening, loss of scales, uni-or bilateral exophthalmia, haemorrhages of the skin especially in the base of fins and abdominal distention. The common postmortem lesions were congestion of internal organs. The histopathological changes were hydropic degeneration of hepatocytes, deplation of haemopoietic Assiut Vet. Med. J. Vol. 52 No. 109 April 2006 elements and necrosis of kidney tissues. The isolated S. faecalis found to be sensitive to a wide range of antibiotic and oxytetracycline and amoxicillin were the drugs of choice.