INTRODUCTIONRickettsia rnooseri. the etiologic agent of murine typhus, is transmitted from its natural rodent host to man by the oriental rat flea, Xenopsylla cheopis.' When these rickettsiae are imbibed in an infectious blood meal, they propagate in the midgut epithelium of the flea and are excreted in the feces, which thus becomes the actual vehicle of infection.'-' A similar sequence of events occurs in the transmission of epidemic typhus rickettsiae by the infected human body louse. Adult rat fleas may under normal circumstances live for many months. Infection of the flea with R . tnooseri can occur only during the parasitic adult stage, and the life-span of the flea is not shortened by the infection.5Although electron microscopic features of the infection of the body louse by Rickettsiae prowazeki have been examined in several studies,5-12 no comparable investigations have been made of rickettsia1 infection in oriental rat fleas. Indeed, ultrastructural studies on the flea midgut epithelium have so far been limited to a short report by Reinhardt et aI.l3 on the midgut of the oriental rat flea and two other species and to an earlier description by Richards and Richards I ' of a novel beaded layer that forms the basement lamina that underlies the midgut epithelium of the flea, Ctenophthalmus.Herein, we intend to provide further information on normal midgut epithelium of X. cheopis and to elucidate some of the ultrastructural features of the infection of this species by murine typhus rickettsiae. MATERIALS AND METHODSThe Wilmington strain l5 of R. tnooseri used in the present study represents the eighth passage in the yolk sacs of developing chick embryos since it was received in this laboratory in 1946. The stock pool of this strain was a 50% suspension of infected yolk sac in sucrose-PG,13 aliquots of which were stored at -70" C.Our colony of Xenopsylla cheopis, originally obtained from the United States