Efficacy and residues of oxolinic acid in tissues of experimentally inoculated Oreochromis niloticus fish with Yersinia ruckeri (Y.rucken) have been investigated. Results of the in-vitro stuy revealed that the used bacterial isolate was sensitive to oxolinic acid. The in-vivo treatment test indicated a high effectiveness of oxolinic acid when given orally in the diet at a dose rate of 10 mg/kg body weight for ten successive days. The haematological inestigations in experimentally infected fish proved a decrease in some blood parameters namely, RBCs, WBCs count, Hb content and P.C.V. volume pre-treatment with oxolinic acid, but they returned to their normal values post-treatment. The residual analysis indicated the presence of oxolinic acid residues in the muscles, kidney and liver of treated fish up to 15, 20 and 25 days post-treatment, resppectively.