Engorged larvae of Leptotrombidium pallidum were collected from a wild rodent, Apodemus speciosus, and reared in a plastic container fed with fresh eggs of colembolla, Sinella curviseta, under moist conditions at 25 degrees C. Fully developed adults were separated into individual containers and a proven male was paired with a suspected female. Of the pairings, 38 produced larvae and 4 infected colonies were established. Females in these colonies were shown to be infected with Karp type of Rickettsia tsutsugamushi (Rt) by the method of isolating Rt from individual mites using mouse passage. Males were negative for Rt. Vertical transmission of Rt was monitored in each infected colony. The female of parent generation (P) produced F1 and F2 generations. At F1 generation, 11 males and 12 females maintained until production of eggs and all adults were positive for Rt by detection from individual mites. By sibmating of F1 adults, 11 males and 14 females developed at F2 generation and all males and females were proven to be positive for Rt. From 4 pairs of infected males and infected females at F1 generation, 2, 5, 21 and 35 larvae were collected as samples, respectively, and all larvae were positive for Rt by individual detection. Spermatophores from infected males at F2 generation were examined for Rt and none of 29 spermatophores were positive for Rt. From these results, the transmission rate of Rt from P to F1 and from F1 to F2 was 100% through eggs from infected females but not through spermatophores from infected males.