Three new species of the genus Mesodolylaimus Andrassy, 1959 are described from Livingston Island, South Shetland Islands, Antarctica. Mesodorylaimis chipevi sp. n. is characterised by having a long body (L=1.8-2.2 mm); tail tip cylindrical, rounded; deeply folded and thick cuticle in the vulval region; M. antarcticus sp. n. by its medium body size (L=1.3-1.6 mm), Lip region off set by a shallow depression, odontostyle 13-15 mum, transverse vulva, pars refringens vaginae consisting of triangular screrotizations, tail first conoid then more or less uniformly tapering to a spicate or narrowly rounded terminus; M. masleni sp. n by its medium body size (L=1.5-1.8 mm), Lip region continuous with the body outline, odontostyle 15-17 mum, vulva transverse; presence of wrinkles and folds around vulva; pars refringens vaginae consisting of two well developed triangular arcuate sclerotizations, long tail (91-117 mum), ventrally straight, dorsally convex and subcylindroid. Some additional data on M. imperator are provided. A feature common to all Antarctic species of Mesodorylaimus is the presence of ventral papillae in the anterior and posterior parts of the body. A key to the species of the genus occurring in Antarctica is provided.
