Rickettsia tsutsugamushi was isolated from mite larvae collected from 37 rodents and from black plastic plates in the Ban Pong area of Rajburi and at Prong Ma Deau in the Muang district of Nakorn Pathom, 2 known endemic areas of scrub typhus in Thailand. Of 17 pools of Trombicula deliensis inoculated into mice, 6 (35. 3%) yielded richettsiae. No other local mites were incriminated as vectors of the Rickettsia. The 6 strains of R. tsutsugamushi were recovered from mites collected from 2 Bandicota indica, 2 B. bengalensis, one Tupaia glis and one Rattus sp. Cross-challenge tests showed that the strains were identical to the Karp strain of R. tsutsugamushi. Strains of R. tsutsugamushi were also isolated from the tissues of 6 of the 37 rodents examined, but the rate of recovery (16. 2%) was lower than from pools of the mite (35. 3%).