ABSTRACT: Aggregata bathytherma sp. nov. is described from the digestive tract of Vulcanoctopus hydrothermalis, a deep-sea octopus recently discovered associated with hydrothermal vents in the northeast Pacific Ocean. Oocysts typically are spherical in shape, sometimes irregular, 163 to 356 µm in length, and 219 to 313 µm in width. Each oocyst contains from 50 to over 200 sporocysts. Sporocysts measure 27 to 32 µm in longest diameter. The cyst wall is smooth and 1 µm thick. Each sporocyst typically contains 14 to 17 sporozoites, 49 µm in length. Histological lesions associated with the presence of A. bathytherma include rupture of the basal membrane and detachment of the epithelial cells. In heavily infected areas, most of the tissue of the host digestive tract is replaced by parasites. A. bathytherma is the first Aggregata species described from a host that lives in association with hydrothermal vents, and the third species of Aggregata from eastern North Pacific waters.