Eperythrozoon coccoides, an epierythrocytic organism that causes a mild haemolytic anaemia in laboratory and wild mice, currently is thought to be a rickettsia. To determine the relationship of this agent to other haemotrophic bacterial parasites, the 16S rRNA gene of this organism has been sequenced and it is shown by phylogenetic analysis that this wall-less bacterium is not a rickettsia but actually is a mycoplasma. This mycoplasma shares properties with and is closely related to the other uncultivated mycoplasmas that comprise a recently identified group, the haemotrophic mycoplasmas (haemoplasmas). The haemoplasma group is composed of former Eperythrozoon and Haemobartonella species as well as newly identified haemotrophic mycoplasmas. Haemoplasmas parasitize the surface of erythrocytes of a wide variety of vertebrate animal hosts and are transmitted mainly by blood-feeding arthropod vectors. Because both primary infections and chronic latent infections caused by this bacterium have been observed in many laboratories and this bacterium has been the subject of much experimental work, considerable information exists about this haemotrophic mycoplasma that may be applicable to other haemoplasmas. It is proposed that Eperythrozoon coccoides be reclassified as Mycoplasma coccoides comb. nov. A Request for an Opinion is submitted to the Judicial Commission of the International Committee on Systematics of Prokaryotes regarding this reclassification.