Haemogregarina mansoni Sambon and Seligmann 1907, a parasite of the coachwhip snake (Masticophis flagellum) is redescribed on the basis of sporogonic stages obtained in the experimental vector Aedes aegypti and designated as Hepatozoon mansoni from hosts collected in north Florida. Gamonts average 14.7 × 4.7 (13–16 × 4–6) and are not recurved, with nuclei situated in the second quarter of the gamont. Erythrocyte cytoplasm is often thin, appearing partially dehemoglobinized, or contracted into a central mass with infected erythrocytes always distorted, longer, and more slender than uninfected cells. Sporogony occurred within the head and thorax of A. aegypti. Oocysts were spherical to ovoid, 144.2 × 126.1 (79–198 × 69–178), containing on average 29.2 (7–64) sporocysts. Sporocysts were spherical to ovoid, 33.1 × 29.8 (19–48 × 18–44), with 20.2 (12–32) sporozoites contained within. Experimental infection in Tantilla relicta produced gamonts that did not differ from those in M. flagellum, but dehemoglobinization and cytoplasmic contraction of the host erythrocyte did not occur, and persistent merogonic stages were not present in the tissues.