Haemogregarina macrochelysi n. sp. (Apicomplexa: Haemogregarinidae) of the alligator snapping turtle, Macrochelys temminckii, is characterized by slender, recurved gamonts 29–35 × 3–4.5 μm, in which the anterior limb comprises 48–54% of the total length. The gamont nucleus, 5–7.5 × 2–5 μm, is situated at approximately midbody of the gamont. Meronts typical of Haemogregarina occupying erythrocytes have 3-8 small, compact nuclei and are 13–17 × 4.5–9 μm. Erythrocytic meronts that contain larger, nearly square or rectangular nuclei become rounded, and then undergo 7 or more nuclear divisions, which produce very large, usually ovoid to rounded meronts that may contain up to 150 nuclei or more within the thinly stretched host erythrocyte membrane. In tissues of the Placobdella spp. leech vectors, merogony occurs directly from sporozoites, forming merozoites that presumably are infective for the turtle host.