- Y. ruckeri causes enteric redmouth disease or yersiniosis; a serious bacterial septicaemia of freshwater and marine fishes. This study was carried out to investigate Y. ruckeri infection among O. niloticus (Nile tilapia) and C. gariepinus (African catfish) at Sohag Governorate. Ninety-three samples of O. niloticus and eighty-seven samples of C. gariepinus were collected from different localities at Sohag Governorate during the period from March 2014 to March 2015. Fish samples were subjected to clinical, post-mortem examination and bacteriological examination. The liver, kidney and spleen were chosen for bacteriological examination. The suspected isolates were characterized by cultural and morphological characters, some conventional biochemical tests and API 20E system. Five isolates were characterized as Y. ruckeri [3 isolates (Y1-Y3) from O. niloticus (3.2%) and 2 isolates (Y4-Y5) from C. gariepinus (2.3%)]. The phenotypic characterization of these isolates revealed that they were homogenous except in Voges-Proskauer test and gelatine liquifaction, the similarity between them was ranged from 92.9 to 100% and they belonged to Biotype 1 of Y. ruckeri.