Orientia tsutsugamushi is a causative agent of scrub typhus, and a Gram-negative obligate intracellular bacterium that is transmitted to humans by the bite of infected Leptotrombidium mites. In Korea, the incidence of scrub typhus has increased remarkably in recent years. A total of 6022 cases of scrub typhus were reported in 2007, and the incidence of scrub typhus infection was 13.2 cases per 100 000 persons. To develop a national strategy for the control and prevention of this disease that is based upon scientific data, we surveyed the infection rate and analysed the distribution of O. tsutsugamushi genotypes among wild rodents and engorged chigger mites in Korea. The specimens used in the present study were obtained from rodents because rodents have been recognised as a natural reservoir of O. tsutsugamushi and a host of chigger mites. At monthly intervals between August 2005 and July 2007, Sherman live traps were used to capture wild rodents living in four regions of Korea. Specimens were obtained from two areas in southern Korea, Chogye in Chollanam province and Ganjeon in Gyeongsangnam province, and from two areas in the middle region of Korea, Jangan and Songsan in Gyeonggi province. Chigger mites were collected from the wild rodents that had been captured. The 56-kDa genes of O. tsutsugamushi were collected from the blood or spleens of the wild rodents, and were detected from the pools of 30 chigger mites by nested PCR amplification, which was carried out as described by Furuya et al. [1]. In the present study, genotypes were determined by PCR-RFLP using DdeI. Analysis of the nucleotide sequence homology among the nested PCR products was performed using the CLUSTALX (http://clustal.org) and Tree Explorer (Tamura; http://evolgen.biol.metrou.ac.jp/TE/TE_man.html) programs in order to confirm the genotype in cases that could not be determined by RFLP. The rates of positivity among wild rodents from Chogye, Ganjeon, Jangan and Songsan, were 4.5% (12 ⁄266), 5.0% (10 ⁄201), 12.4% (25 ⁄202) and 5.0% (13 ⁄258), respectively, as shown in Table 1. To monitor the infection rate in mites, we calculated the MPR (minimum positive rate) using the equation: number of positive mites ⁄total number of mites examined. The MPRs were 0.6% (6 ⁄1071), 0.3% (12 ⁄4215), 0.9% (26 ⁄2893) and 1.3% (85 ⁄6752) in Chogye, Ganjeon, Jangan and Songsan respectively. The rates of positivity for O. tsutsugamushi were 6.5% (60 ⁄927) and 0.9% (129 ⁄14 931) in the wild rodents and chigger mites, respectively. In a previous study, Ree et al. [2] analysed the incidence of O. tsutsugamushi by IFA and PCR using 4315 individual mites collected from eight localities in Korea and reported that the infection rate was 1.5%, which was slightly higher than the result reported in the present study. From these results, we were able to determine that there is a high risk of scrub typhus infection in the natural environment of Korea. The Boryong genotype was the most common, at a rate of 76.7% in wild rodents and 82.9% in chigger mites, followed by the Gilliam (10%), Pajoo (8.3%) and Karp (5%) genotypes in the wild rodents and the Karp (11.6%), Pajoo (3.1%), Yoncheon (1.6%) and Kawasaki (0.8%) genotypes in the chigger mites. The Kato genotype was not detected in any of the specimens. The geographical distribution of the Boryong genotype was slightly higher in southern Korea than in mid-Korea, while the Gilliam, Karp and