The present study was carried out on 200 broiler chickens which had respiratory troubles on different farms of different districts at Sharkia Governorate, to throw a spot light on the clinical and microbiological investigations as well as prevalence rate of ORT infection in broiler flocks. Bacteriological examination revealed that 24 isolates (12%) of ORT were isolated from the examined birds. ORT was isolated from trachea, lungs, air sacs and sinuses in a percentage of 10.5%, 8%, 5.5% and 1% respectively. No ORT was isolated from liver. Serotyping of ORT using AGPT proved that all the isolates were belonged to serotype A. Antibiogram studies showed that all the isolates were highly sensitive to Amoxycillin, Ampicillin, Oxytetracyclin, and resistant to Gentamicin, Danofloxacin and colistin. Experimental infection with ORT evoked respiratory signs with mortality rate reached to 5%.