Vertical transmission of Orientia tsutsugamushi (Hayashi), the etiologic agent for scrub typhus, was studied in two lines of naturally infected Leptotrombidium deliense Walch. In one line of mites originating from a single adult (V3M), the rate of filial transmission was 100% for the first two laboratory generations, but declined to 86.6% in the third laboratory generation. The vertical infection rate in this line of mites was 100% for the parental generation, but declined to 95.6% for the F1 generation and 88.6% for F2. The transmission of O. tsutsugamushi in another line of L. deliense (V3F) was less efficient than mites originating from V3M. In the initial laboratory generation of V3F a filial transmission rate of 100% was recorded. However, none of the F2 generation of this line transmitted rickettsiae to mice (Mus musculus L.), resulting in a filial transmission rate of 0%. Transmission of O. tsutsugamushi to mice by progeny from cohort larvae originally from the same adult (V3F) was also studied in the laboratory and these were found to be relatively poor transmitters of rickettsiae. The filial infection rate of F2 larvae was 60%, F3 was 88.8%, and F4 was 55.9%. The biology of infected L. deliense was studied and compared with uninfected mites reared under the same laboratory conditions. The results showed that infected female L. deliense laid approximately the same or more eggs as uninfected adults. The rate of development of the progeny of infected L. deliense was not significantly different from uninfected mites.