a chronic infectious disease of poultry, affecting the respiratory organs and resulting in emaciation and inability to produce eggs. It was first described in the USA by J. Delaplane and H. Stewart in 1943; in the USSR it was discovered and described by la. R. Kovalenko and A. la. Fometskaia in 1959. The causative agent of the disease in fowl and turkeys is Mycoplasma gallisepticum. The source of the infectious agent is a diseased bird. Symptoms of the disease are decreased appetite, rhinitis, conjunctivitis, and tracheal rales. Diagnosis is made in the laboratory, taking into account epizootic and clinical data. There are no effective means of treatment. Preventive measures include raising only the young of healthy birds and strictly observing veterinary-sanitary and zoohygienic rules of maintenance.