Two cases of Mycoplasma gallisepticum, infection in different avian species in backyard gamebird operations in Slovenia were investigated. in the first case, M gallisepticum, was associated with severe respiratory disease with almost 20 per cent mortality in pheasants, whereas the infection was less pathogenic for chickens and turkeys reared at the same site. The M gallisepticum isolates from pheasants had a unique pMGA gene sequence containing a repeat of 12 nucleotides, and they contained only small amounts of the cytadhesins mGC1 and MGC3 and no PvpA protein. However, they expressed some typical M gallisepticum proteins and several proteins which were immunogenic for pheasants, chickens and turkeys. A strain of M gallisepticum, isolated from the sinus of a pheasant was highly pathogenic for chicken embryos. in the second case, the M gallisepticum strain that was associated with respiratory disease and mortality in peafowl also affected chickens. M gallisepticum strain ULB 992 was isolated from the infraorbital sinus of a dead peafowl. The ULB 992 strain synthesised a small amount Of MGC3, a truncated form Of MGC1 and lacked PvpA. However, it expressed several proteins which were immunogenic for the birds infected with M gallisepticum at both gamebird operations.