The neocallimastigomycetes are obligate anaerobic fungi living in the rumen and cecum of herbivorous animals such as cows, sheeps, horses, elephants and kangaroos. They digest ingested plant in extracellular cellulosome using some enzymes (e.g. cellulase) originated by lateral gene transfer from bacteria. The genome has an unusually low GC content (13-22%).

The thalli are monocentric or polycentric. The zoospore possesses single to many flagella. There are no centriole and lipid globule in zoospore. They have radiating microtubular roots orginated from the spur connected to basal body. The mitochondria lost genome, cristae and the ability of aerobic respiration, and reduced to hydrogenosomes, which form hydrogen and carbon dioxide from pyruvate and malate during fermentation of carbohydrates. Asexual reproduction by means of zoosporogenesis.