Abstract: The present invention relates to a live cytadherence-deficient M. gallisepticum strain that does not express at least two of three proteins, the Gap-A molecule, crnA protein, and the 45 kDa protein, expressed by wildtype M. gallisepticum Strain R and its use as a vaccine for preventing and protecting birds, especially chickens and turkeys against the respiratory diseases attendant with wildtype Mycoplasma gallispeticum infection. The invention also relates to the use of the vaccine as a vector for the delivery of genes encoding protective antigens from other bacterial and viral avian pathogens, such as avian influenza virus. There is also disclosed a method for identifying the attenuated cytadherence-deficient M. gallisepticum Rhigh or a strain thereof.