ABSTRACT. We recently reported the isolation of a leptomyxid ameba from the brain of a mandrill baboon that died of meningo-encephalitis. Based on light and electron microscopic studies, animal pathogenicity tests, and immunofluorescence patterns, we conclude that our isolate differs fundamentally from the other two amebas (Leptomyxa and Gephyramoeba) included in the Order Leptomyxida. We therefore created a new genus, Balamuthia, to accommodate our isolate and described it as Balamuthia mandrillaris to reflect the origin of the type species. Briefly, B. mandrillaris is a pathogenic ameba that causes amebic encephalitis in humans and animals. It has trophic and cyst stages in its life cycle, and is uninucleate with a large vesicular nucleus and a central nucleolus. Mature cysts have a tripartite wall consisting of an outer loose ectocyst, an inner endocyst and a middle mesocyst. Unlike Acanthamoeba and Naegleria, the other two amebas that cause amebic encephalitis in humans, Balamuthia will not grow on agar plates seeded with enteric bacteria. However, Balamuthia grows on a variety of mammalian cell cultures and kills mice following intranasal or intraperitoneal inoculation. Based on immunofluorescence testing, 35 cases of amebic encephalitis in humans and three in other animals have been identified worldwide as being caused by Balamuthia.