The Mammoth Cave system includes more than 175 kilometers of explored passages in Mammoth Cave National Park, Kentucky. Although biologists have explored the caves intermittently since 1822, the inventory of living organisms in the system is still incomplete. The present study lists approximately 200 species of animals, 67 species of algae, 27 species of fungi, and 7 species of twilight-zone bryophytes. The fauna is composed of 22% troglobites, 36% troglophiles, 22% trogloxenes, and 20% accidentals, and includes protozoans, sponges, triclads, nematodes, nematomorphs, rotifers, oligochaetes, gastropods, cladocerans, copepods, ostracods, isopods, amphipods, decapods, pseudoscorpions, opilionids, spiders, mites and ticks, tardigrades, millipedes, centipedes, collembolans, diplurans, thysanurans, cave crickets, hemipterans, psocids, moths, flies, fleas, beetles, fishes, amphibians, birds, and mammals. The Mammoth Cave community has evolved throughout the Pleistocene concomitantly with development of the cave system. The troglobitic fauna is derived from 4 sources: (1) troglobite speciation in situ in the system itself; (2) dispersal along a north Pennyroyal plateau corridor; (3) dispersal along a south Pennyroyal plateau corridor; and (4) dispersal across the southwest slope of the Cumberland saddle merokarst.