Blood from 4 wild-trapped bobcats (Lynx rufus rufus) with naturally occurring infection by an intraerythrocytic piroplasm morphologically indistinguishable from the piroplasm form of Cytauxzoon felis was inoculated parenterally into domestic cats. None of the bobcats had signs of disease at the time of capture or subsequently, but all remained parasitemic. Erythroparasitemia developed in all 4 of the domestic cats inoculated with parasitemic bobcat blood. One of the domestic cats later died and had gross and histologic lesions characteristic of those reported in naturally occurring and experimentally induced fatal cytauxzoonosis of domestic cats. The other 3 cats remained parasitemic but otherwise appeared healthy. One of these cats was subsequently inoculated with virulent Cytauxzoon of domestic cat origin and it died within 14 days. Two of the bobcats, including the one whose blood induced fatal cytauxzoonosis in the domestic cat, were similarly inoculated but remained without apparent ill effects. Schizogenous tissue forms of Cytauxzoon organisms were not found in tissues from any of the 4 bobcats or in tissues from the 2 parasitemic domestic cats that were not inoculated with the virulent organism. The results demonstrated that the bobcat is a natural host for the organism that causes fatal cytauxzoonosis in domestic cats and provides presumptive evidence that the naturally occurring bobcat piroplasm may be the intraerythrocytic form of C felis.