A serological study of antibodies to Ehrlichia risticii was carried out on 10 species of wild and domestic mammals found on or near 21 horse farms in an area of the USA in which Potomac horse fever is endemic. No antibodies were found in 133 peridomestic rodents (Norway rats and house mice), nor in 108 wild rodents (white-footed mice and meadow voles) captured on farms. Three of the six domestic animal species examined, cats, pigs and a goat, showed serological evidence of exposure to E risticii. Seropositive animals were detected on three of the 21 premises. The eight seropositive cats (of 48 cats tested) were on two farms, and the three seropositive pigs (of 14 tested) were all on one farm which lay some 3 km from where the one seropositive goat (of three tested) was found. None of the 79 dogs, 75 cattle and seven sheep tested had antibodies to E risticii. The significance of these findings is discussed in the light of current understanding of the transmission of Potomac horse fever and of the epidemiology of other related ehrlichial diseases.