American canine hepatozoonosis is an emerging, tick-transmitted infection of domestic dogs caused by a recently recognized species of apicomplexan parasite, Hepatozoon americanum. The known definitive host of the protozoan is the Gulf Coast tick, Amblyomma maculatum. Presently recognized intermediate hosts include the domestic dog and the coyote, Canis latrans. Laboratory-reared larval or nymphal A. maculatum can be infected readily by feeding to repletion on a parasitemic intermediate host; sporogony requires 35–40 days. Transmission of infection to the dog has been produced experimentally by oral administration of mature oocysts or oocyst-containing ticks. Canine disease follows experimental exposure in 4–6 weeks and is characterized by systemic illness, extreme neutrophilic leukocytosis, muscle and bone pain, and proliferation of periosteal bone. Histopathological findings include multifocal skeletal and cardiac myositis associated with escape of mature merozoites from within the host-cell environment. There is also rapid onset of periosteal activation and osteogenesis and, less frequently, glomerulopathy and amyloidosis. Sequential stages of development of H. americanum in both the dog and the tick have been elucidated. Gamonts potentially infectious to ticks have been observed in peripheral blood leukocytes of the dog in as few as 28 days after exposure to oocysts. Young coyotes experimentally exposed to a canine strain of H. americanum acquired disease indistinguishable from that of similarly exposed young dogs.