Yersiniosis caused by yersinia ruckeri was studied in Oreochromis niloticus fish farm in Egypt. Recurrences of the disease condition occurred in summer season and correlated with improper management represented by poor water quality and continuous farming operations. Symptomatic and asymptomatic infections were recorded in naturally and experimentally infected fish where the principal signs of the disease were congestion of the head region, bases of the fins and internal organs. Naturally ,the percents of recovery of Y. ruckeri from clinically infected, asymptomatic dead and apparently healthy fish as well as from their corresponding pond water samples reached 88%, 76%, 56% and 100%, 80%, 40% respectively. Experimentally, cumulative morbidity and mortality presents of intraperitoneally infected fish groups with y. ruckeri at 22+-1c and reached 60%, 100% and 25%, 55% respectively, however, these percents reached 35%, 95%, and 10%, 40% in fish groups kept in immersion with Y. ruckeri at the same temperatures respectively. Histopathological changes were mainly inflammatory and degenerative in nature. The prevention and principals of disease control were also discussed.