Mycoplasma imitans (Mim) is a close relative of Mycoplasma gallisepticum (Mg), but has been isolated from ducks, geese and partridge. In order to investigate its potential pathogenicity for turkeys a UK isolate of Mim from a partridge with sinusitis was first passaged through turkey poults and then assessed for pathogenicity in turkey embryo tracheal organ cultures (TOCs) and in one-day-old turkey poults with or without turkey rhinotracheitis virus (TRTV). Mim appeared to gain virulence on passage through poults and this was confirmed by an increased ciliostatic effect in TOCs. In single infection in poults the organism caused mild upper respiratory disease but in dual infection with TRTV there was a significant increase in clinical signs and lesions. The mycoplasma was only isolated from upper respiratory tract in single infection but was recovered also from lung and airsacs in the presence of the virus. There was also a higher humoral immune response in the dual infection than the single Mim infection and there were some cross-reactions with commercial Mg stained antigen in the rapid serum agglutination test.