In 1994 and 1995, seven cases of Human monocytic ehrlichiosis were reported in Indiana; six cf these were from southern counties. To test whether Ehrlichia cha ffeensis is present in native ticks, adult ticks representing two species, Dermacentor variabilis and Amblyomma americanum were collected in southern Indiana during the first week of May, 1995 and tested by PCR. A total of 510 D. variabilis ticks were collected and placed into 102 pools (5 ticks per pool). A total of 430 adult A. americanum ticks were collected and placed into 88 pools of 5 ticks or less.D. variabilis ticks were used to optimize the isolation of PCR amplifiable DNA and determine the minimum number of bacterial cells detected. A modified CTAB-phenol method permitted the detection of as few as 100 bacterial cells. The optimal amount of isolated DNA for a PCR amplification was determined to be 2.7 pg of total nucleic acid.The 88 pools of adult A. americanum were subjected to DNA isolation, PCR amplification, and Southern analysis to determine the presence of E. chaffeensis bacteria. Using the 16S rRNA gene from E. chaffeensis, with species specific primers Hut and HE3. Of the 88 pools (430 ticks; 21 were determined to be positive for the presence E. chaffeensis bacteria. Thus, minimal field infection rate for adult A. americanum ticks in Southern Indiana was 4.88%. This calculation is based upon the assumption that at least one tick was positive in each positive pool. / Department of Biology