This communication describes a new species of Mononchus which was found at the Experimental Station of Monte Alegre (Ibiti), State of São Paulo. Species of Mononchus are of interest because they are believed to be predators of other nematoids. The predatory habit of the new species was observed in the laboratory. Assuming that its behaviour is the same in the field, we may consider the species as an ally of man in controlling populations of nematoids in the soil. M. ibitiensis differs from its nearest related species M. muscorum by its smaller pharynx ; by smaller dortooth, opposited by four ribs, each of which has six denticles and by the presence of three caudal glands connected with a pore without valve.