Despite much recent interest in the middle ear and hearing of subterranean mammals, there is very little information in the literature regarding the middle ear apparatus of tuco-tucos (Rodentia: Ctenomyidae). In this study, the middle ear apparatus of Ctenomys sociabilis was dissected and is described for the first time. The middle ear structures of this species proved to be very similar to those of other caviomorph rodents; for example, in the bullet-shaped malleus head and in the fusion of the malleus and incus. The caviomorphs represent a rather conservative group in this respect. The m. stapedius is not present in C. sociabilis—loss of middle ear muscles is a common trend among fossorial mammals, but this particular feature has been reported in many other members of the superfamily Octodontoidea. Although the middle ear apparatus of C. sociabilis includes features consistent with the fossorial paradigm, some of which might be interpreted as low-frequency adaptations, it is not obviously specialized relative to other caviomorphs in this respect.