Abstract: Ornithobacterium rhinotracheale infection is a contagious disease of birds that causes respiratory distress, mortality, growth decrease and egg production drop. This study was conducted to determine the seroprevalence of O. rhinotrachealae infection in a number of broiler flocks and several flocks of a broiler breeder farm in Mashhad, Khorasan province located in Northeast of Iran. A total of 985 and 211 blood samples were taken from a broiler breeder farm and 13 broiler flocks, respectively. The broiler breeder samples were collected from 6 different flocks of the same farm with the age ranging from 11 to 53 weeks old during one and a half year period. The age of broiler flocks subjected to the sampling ranged from 35 to 55 days old. The serum samples were assayed for antibodies to O. rhinotracheale by a commercial Elisa kit (IDEXX). Result showed that 206 out of 211 serum samples were positive to antibodies to O. rhinotrachealae (97.63%) and the remaining 5 samples were suspicious. In the broiler breeder flocks, 970 out of 985 serum samples were positive (98.48%). The rest of 15 samples were suspicious. In broiler breeder flocks, the highest numbers of positive sera were found in the production period. In conclusion, the results of this study indicated that the prevalence of O. rhinotracheale antibodies is high in the broiler and broiler breeder flocks. Due to the economical importance of O. rhinotracheale infection and its high prevalence it might be necessary to design and implement a prevention and control plan to tackle this emerging infection

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