The role of lone star ticks as vectors for Rocky Mountain spotted fever (RMSF) remains poorly described. We compared the entomological inoculation rates (EIRs) for Rickettsia spp. for representative sites in Missouri and Kansas, states that frequently report RMSF each year. Hostseeking ticks were collected during 2006 and pooled tick homogenates analyzed by polymerase chain reaction to detect probable R. rickettsii, with confirmation for multiple gene targets performed on individual ticks from pools that screened positive. Of 870 adult and nymphal lone star ticks, Amblyomma americanum (L.), 0.46% contained DNA of Rickettsia rickettsii. Interestingly, two of these positive ticks were concurrently infected by R. amblyommii. More than 90% of lone star tick pools contained R. amblyommii DNA. Of 169 dog ticks that were analyzed, none were infected by R. rickettsii. The entomological inoculation rate for spotted fever group (SFG) rickettsiae within lone star ticks was an order of magnitude greater than that for dog ticks. We conclude that lone star ticks may be epidemiologically significant vectors of Rocky Mountain spotted fever and of spotted fever group rickettsiae.