A 4 year old male castrated domestic short hair presented to Cornell University Hospital for Animals for severe anemia caused by Mycoplasma haemofelis. M. haemofelis is one of the hemotrophic mycoplasmas that affect the cat. The hemotrophic mycoplasmas were formerly known as Hemobartonella and were considered to be rickettsial organisms. Recent PCR findings show that it is closely related to mycoplasma organisms rather than rickettsial organisms. Although the disease caused by this organism has been recognized in the US since 1953 there are still several aspects that remain unclear. The exact mode of transmission along with the exact pathogenesis of the anemia has still not been completely worked out. What is known about M. haemofelis is that it is an epierythrocytic parasite that can cause severe hemolytic anemia in cats.