The presence of Mycoplasma haemomuris (haemoplasma) in blood samples collected from specific pathogen-free (SPF) laboratory rats bred in Japan was reported. Its presence was examined in Fischer 344, Sprague-Dawley (SD), and Wistar rat strains of both sexes by real-time PCR. All strains were positive for M. haemomuris infection. The 16S rRNA gene of M. haemomuris strain detected in the animals was amplified using end-point PCR. Only the entire nucleotide sequence of 16S rRNA gene of a mycoplasma strain detected in SD rats was determined and compared to those of other haemoplasmas. Our investigations suggest a wide M. haemomuris infection among the SPF rats purchased from commercial breeders in Japan.