The life history of the protozoan parasite Ascogregarina taiwanensis in mosquito larvae (Aedes albopictus, collected in southern Taiwan) was shown to consist of two consecutive stages--intracellular and extracellular. Light microscopy showed that most trophozoites moved into the Malpighian tubules and developed into giant trophozoites during the first day pupa. The locomotion may be associated with bristle-like ridges of the trophozoite. The stage for sexual reproduction, i.e., the gamete, was then formed by segmentation of the giant trophozoite and twisting off the anucleate extremities of the body. Sexual reproduction occurred via fertilization by fusion of two resulting gametes, presumably two opposed sexes. The fused gametes finally generate the formation of the gametocyst, within which oocysts develop by budding from the cytoplasmic mass. This type of sexual reproduction has not been reported previously in any gregarine protozoa. We here proposed it as a new hypothesis for further elucidation of the protozoan reproduction.