We studied, under experimental conditions, the life cycle of Hepatozoon erhardovae in the tropical rat flea, Xenopsylla cheopis, light microscopically on coloured semi-thin sections. Fleas killed and prepared on the day of infection show stomachs filled with mice erythrocytes. Some monocytes are parasitized or show empty envelopes of gametocytes. The latter perforate the wall of the stomach, leaving behind empty parasitophorous vacuoles, and they migrate to the fat-body cells of the flea where they differentiate sexually and where they stay during the whole of their further sexual development. On days 2 and 3 post infection (p.i.) microgametes were observed with one flagellum each, as well as macrogametes in one and the same host cell. Fertilization is induced by gametogamy. During the various divisions of the nuclei on the days 8-12 p.i. droplet-like evaginations are formed on the pellicle of the oocyst from the peripherical chromatin condensations. Between days 12 and 14 p.i. the sporoblasts develop while the oocysts diminish in size. Between days 18 and 20 p.i. 16 sporozoites and a large residual body are differentiated in each sporoblast and surrounded by the sporocyst wall. The oocyst wall is preserved and forms sporocyst balls that are set free when the flea abdomen is pressed. The flea thus presents a highly infective vector; the intermediate host is infected after eating the flea.