Haemogregarina crocodilinorum Börner 1901 is redescribed from the blood of the American Alligator (Alligator mississippiensis). Gametocytes occurred in erythrocytes and erythrocytic schizonts contained about eight merozoites (range, 6--12). Gametogony and sporogony occurred in the gut of the leech, Placobdella multilineata and intracellular, unisporocystic oocysts were found within epithelial cells of the leech's intestinal wall. Presumed sporozoites were present in the intestinal contents. Attempts to transmit the parasite via leeches failed. The haemogregarine was widely distributed in southern United States and was found in 77 (59%) of 130 alligators examined. A comparison of this parasite is made with other haemogregarines reported from crocodilian reptiles.