M. gallisepticum, M. synoviae and M. meleagridis are pathogenic strains of avian mycoplasma. M. gallisepticum and M. synoviae affect the chicken and turkey while M. meleagridis is associated with disease in the turkey. The tissues affected and the lesions are described and the influence on disease production of certain factors is considered. They include the age of the host, route of infection, numbers and virulence of organisms and predisposing and debilitating factors. The mechanism of pathogenesis is also indicated and reference made to a method of determining virulence.