Mycoplasmaa-induced arthritis of poultry under field conditions is almost entirely caused by Mycoplasm synoviae (Ms) infection. Experimentally, Mycoplasma synoviae, Mycoplasma gallisepticum, Mycoplasma meleagridis and Mycoplasma iowae have all been shown to cause arthritis, tenosynovitis and lesions of periarticular tissue. However, greatest attention has been given to Ms in this respect. The leg joints are most consistently involved, and of these particularly the tibiotarsal-tarsometatarsal and tarsometatarso-phalangeal joints. Acute amd chronic arthritis have been described, and in the later the arthritic lesions may be progressive. The histopathological changes include invasion with heterophils (polymorphs) and later monocytes. Villus formation is shown by the synovium, and lymphofollicular aggregates appear. In severe cases, there is progressive erosion of articular cartilage and there are degenerative changes in underlying bone. Systemic changes include anemia, reticulitis, arteritis, and atrophy of the thymus and bursa of Fabricius. The mechanism of disease production may involve a direct inflammatory response as well as immunoinflammatory complexes.