Of 95 Tsutsugamushi disease case occurring in Yamagata prefecture from 1999 to 2006, four-all women-involved the O. tsutsugamushi Kawasaki serotype. The three major symptoms were fever, exanthema, and eschar present from mid-October to early November. Serodiagnosis by indirect immunofluoresence assay showed elevated IgG and IgM antibody titers against the Kawasaki serotype antigen, with IgM higher than IgG. Nested PCR detected 56-kDa DNA in three of the cases. DNA was amplified in Kawasaki-specific PCR. Two cases for which sequencing was done using nested PCR-amplified DNA showed an identity of 99.8% for the Kawasaki strain (Accession number: M63383). These results confirmed the occurrence of Tsutsugamushi disease infection involving Kawasaki serotype in Yamagata prefecture.