Directed migration of trophozoites from the midgut toward the Malpighian tubules is essential for Ascogregarina taiwanensis (Apicomplexa: Lecudinidae) to complete its developmental cycle within the natural host Aedes albopictus. We have obtained a 275-bp actin cDNA fragment amplified from extracted mRNAs of migrating trophozoites, suggesting the involvement of actin in trophozoite motility. Down-regulation on the migration of the trophozoite was seen in mosquito larvae fed with cytochalasin D, ML-7, and BDM, indicating that myosin, in the form of an actomyosin system, may also be involved in driving motility of the trophozoite. The “protruding apparatus” (PA) formed at the anterior end of trophozoites during the migrating stage had significant deposits of actin by immunofluorescent microscopy. Moreover, PA formation was enhanced in response to elevated levels of 20-hydroxyecdysone (20-HE) in cultures of alimentary canals in which the trophozite was contained. Thus, 20-HE may also promote expression of actin and perhaps myosin simultaneously.