The genus Mundochthonius is represented in the United States by a number of epigean forms usually found in forest litter. Two species of the genus are known from Illinois, M. rossi Hoff and M. sandersoni Hoff (see Hoff, 1949; Hoff & Bolsterli, 1956). The present paper describes a third species, on the basis of specimens found in a cave in southwestern Illinois. This represents the first cavernicolous species of Mundochthonius known from the United States, though Morikawa (1956) has described two cave-dwelling species of the genus from