To continue monitoring the prevalence and distribution ofEhrlichia chaffeensis(Rickettsiales: Rickettsiaeceae) in southern Indiana, a total of 498Amblyomma americanum(L.) ticks (262 adults and 292 nymphs) was collected from five southern Indiana counties during May and June 1998. Ticks were pooled and examined for the presence ofE. chaffeensisusing nested polymerase chain reaction and primers specific for the 16S rRNA gene ofE. chaffeensis.The average minimum infection rate for adult ticks collected in 1998 was 3.8% (ranging from 0 to 7.7% in various counties) as compared with previous average minimum infection rates of 1.6% in 1995 and 4.9% in 1997. None of the pools ofA. americanumnymphs tested positive. In addition, blood samples were collected from 325 white-tailed deer taken in Indiana and 327 taken in Ohio in November 1998. Serum samples were tested for the presence ofE. chaffeensis-like organisms reactive to antibodies using an indirect immunofluorescence assay (IFA). Antibodies were found in deer from six Indiana counties where infection rates ranged from 42.6 to 66.7% and in four Ohio counties where infection rates ranged from 4.4 to 25%. The results of this study reconfirm thatE. chaffeensisis well established in southern Indiana and also provide the first evidence ofE. chaffeensis-like organisms infecting white-tailed deer in Ohio, suggesting the need to survey Ohio ticks for the presence ofEhrlichia.