Abstract: Haemogregarina sinensis requires two hosts in its life cycle: the vertebrate host Trionyx sinensis and the invertebrate host Mooreotorix cotylifer. This paper describes the developmental stages of this parasite in a leech, M. cotylifer, which include gamogony and sporogony. The parasite is ingested by the leech when it sucks a turtle's blood. In the gastric ceca of the leeches, the ingested erythrocytes are digested and release the gametocytes. Then the syzygy association of the male- and femalegametocytes occurs in the intestinal ceca. The male-gametocyte apparently gives rise to 4 male-gametic nuclei, one of which fertilizes the femalegametic nucleus while the others remain as condensed nuclei in the developing oocyst. A mature oocyst produces 8 naked sporozoites. Transmission of this parasite among turtles is most probably the result of ingesting leeches infected with mature sporozoites.