A mycoplasma designated strain 4229T (T = type strain) was isolated in 1984 from the turbinate of a duck in France, and similar strains were isolated from geese in France and from a partridge in England. All of these strains were originally identified as Mycoplasma gallisepticum by immunofluorescence and growth inhibition tests, but subsequent serological and molecular studies indicated only a partial relationship to this species and DNA-DNA hybridization studies revealed only approximately 40 to 46% genetic homology with M. gallisepticum PG31T. In this study morphological, cultural, and physical investigations were carried out on strain 4229T and partridge strain B2/85, after we first demonstrated the similarity between these organisms by performing a restriction enzyme analysis of their DNAs. Both strains had phenotypic properties very similar to M. gallisepticum properties, including the presence of an attachment organelle. As a result of these investigations, the organisms were assigned to the class Mollicutes, the order Mycoplasmatales, and the genus Mycoplasma. They fermented glucose, reduced triphenyl tetrazolium chloride aerobically and anaerobically, and exhibited hemadsorption and hemagglutination, but other biochemical tests were negative. Apart from a serological cross-reaction with M. gallisepticum, these organisms exhibited no significant relationship with any previously described Mycoplasma species as determined by growth inhibition or immunofluorescence tests or with a number of additional serovars and unclassified avian strains. This Mycoplasma taxon therefore appears to be a new species, for which we propose the name Mycoplasma imitans. The type strain is strain 4229 (= NCTC 11733 = ATCC 51306).(ABSTRACT TRUNCATED AT 250 WORDS)