High Quality Content by WIKIPEDIA articles! Blastocystis is a genus of single-celled protozoan parasites belonging to a group of organisms known as the Stramenopiles (also called Heterokonts) that includes algae, diatoms, and water molds. Blastocystis comprises several species, living in the gastrointestinal tracts of species as diverse as humans, farm animals, birds, rodents, reptiles, amphibians, fish, and cockroaches.