From 1981 through 1993, tick infestations and serum antibodies reactive to Ehrlichia chaffeensis, the causative agent of human ehrlichiosis, were monitored among white-tailed deer (Odocoileus virginianus) at Whitehall Experimental Forest, Clarke County, Georgia (USA). Neither ticks nor E. chaffeensis antibodies were detected during the first two years of the study. Infestations of the lone star tick (Amblyomma americanum), a suspected vector of E. chaffeensis, first were noted on deer in 1983. Prevalence and intensity of A. americanum sharply increased from 1985 to 1989, and prevalence was 100% from 1990 to 1993. Antibodies reactive to E. chaffeensis were first detected in 7% of deer sampled in 1986. Antibody prevalence increased to 21% in 1987 and was 100% from 1988 to 1993. This temporal association between the establishment of A. americanum and the appearance of E. chaffeensis antibodies provides evidence to support the concept that A. americanum could be a natural vector of E. chaffeensis. The high prevalence of antibodies among all age classes of deer also reaffirms that white-tailed deer may be sensitive natural sentinels for monitoring the distribution of E. chaffeensis.