SYNOPSIS. Hepatocystis has been found extremely common in a number of species of Formosan mammals, notably the large leafnosed bat (Hipposideros armiger terasensis), the Formosan macaque (Macaca cyclopis), and various subspecies of the red‐bellied tree squirrel (Callosciurus erythraeus). Less often infected were the long‐winged bat (Miniopterus schreibersii) and the Formosan giant flying squirrel (Petaurista grandis). The species occurring in H. a. terasensis is believed to be new, and is named Hepatocystis hipposideri. It causes considerable enlargement (but not other changes) of the host cell, is often amoeboid, and usually has numerous fine pigment granules.