The maculatum agent, Rickettsia parkeri (a member of the spotted fever group rickettsiae), was inoculated into a colony of the lone star tick, Amblyomma americanum, and followed for two tick generations. In addition, guinea pigs were exposed to the agent by direct injection and by feeding infected ticks on them. Eighty (53%) of 150 nymphal A. americanum that were inoculated with suspensions of R. parkeri were positive by hemolymph test and fluorescent antibody test for rickettsial infection when examined as adults. One-month survival of R. parkeri-infected ticks was similar to that of control (noninfected) ticks. Transstadial and transovarial transmission of R. parkeri in the laboratory was demonstrated in A. americanum. When guinea pigs were exposed to the maculatum agent by either direct injection of Vero cell-grown R. parkeri, injection of homogenates of infected ticks, or by feeding of infected ticks, they developed mild fevers and occasional scrotal reactions. These data indicate that R. parkeri can remain viable in lone star ticks for two generations and suggest that guinea pigs may become infected, displaying mild clinical signs.