In this experimental study, our objective was to investigate the macroscopic, microscopic and immunohistochemical findingsdeveloped in the respiratory system due to the co-infection with infectious bronchitis virus (IBV) M-41 and the commensalBordetella avium in the respiratory tract in 15-day-old female Brown-Nick chicks. In this study, a total of 70 non-SPF, healthychicks were randomized into 4 groups. The chicks in Group 1 (n=20) were infected only with IBV-M41 strain; chicks in Group2 (n=20) were infected with IBV-M41 and B. avium; chicks in Group 3 (n=20) were infected only given B. avium. Group 4 (n= 10) were given sterile physiological saline at the same dose and route. The administration of the strains was performed withan intranasal injection. On the 2nd-7th., 9th, 11th, 13th, 15th, and 18th days of the post-inoculation, 2 chicks from theexperimental groups and 1 chick from the control group were sacrificed. In Group 1, the degenerative and inflammatorychanges, which were observed in the 2nd day of infection, disappeared on the 15th day. In Group 2, we noticed bacterialcolonies and mucous exudate containing heterophile leukocytes in the lumens of trachea and larynx. Starting from the 13thday, findings related to recovery e.g., in Group 1 were noticed. In the microscopical examination of the tissues byimmunohistochemistry and immunofluorescent methods, we observed the antigenic distribution in the upper respiratory tractin Group 1 between the 2nd and 5th days and in Group 2 between the 2nd and 9th days. However, lungs were involvedbetween the 5th and 11th days in Group 1 and between the 7th and 11th days in Group 2. In conclusion, we determined thatthe progress of findings was relatively milder in chicks primarily infected with IBV-M41and the findings lasted longer in chicksco-infected with B. avium and IBV caused severe symptoms in chicks in case of co-infection.