Potomac Horse Fever (PHF) is a disease of great concern to many horse owners in the Potomac River area of Maryland and Virginia. It is caused by a rickettsia, Ehrlichia risticii. The involvement of an arthropod vector has been suspected because of the seasonal epidemiology of the disease. This research was an attempt to identify and evaluate potential arthropod vectors. A seasonal activity study of biting arthropods attacking horses in endemic areas of Maryland and Virginia identified five potential vectors: (1) Simulium jenningsi (Diptera: Simuliidae), (2) Stomoxys calcitrans (Diptera: Muscidae), (3) Culicoides obsoletus (Diptera: Ceratopogonidae), (4) C. variipennis, and (5) Dermacentor variabilis (Acari: Ixodidae). These five arthropod species were given status as potential vectors because they were collected feeding on horses just prior to and throughout the PHF season. Simulium jenningsi and D. variabilis have the closest seasonal association with the occurrence of PHF as presented in this study. D. variabilis was determined to have the greatest potential due to its reported association with other rickettsial diseases. A series of laboratory and field studies were designed to examine the potential role of D. variabilis in the transmission of E. risticii. We first attempted to transmit E. risticii by feeding adult D. variabilis collected from an endemic farm on susceptible horses. Other laboratory studies included mouse to horse and mouse to mouse transmission attempts using ticks fed on mice inoculated with E. risticii. A serological survey of 105 trapped field rodents (host of immature D. variabilis) on endemic farms in Maryland showed all specimens collected to be negative for PHF antibodies. These studies and others gave no indication of D. variabilis's involvement in the transmission of the disease in nature. The other species mentioned above were not examined. / Ph. D.