Any of a group of small single-celled organisms, typically less than 30 µm long, that have highly reduced mitochondria called mitosomes, two nuclei, and multiple flagella. Free-living species are typically found in oxygen-deficient environments, such as lake sediments and marshes, while others are anaerobic and live as parasites, including Giardia intestinalis, which inhabits the guts of mammals and causes the disease giardiasis. Diplomonads belong to the excavates assemblage of protists.