Theristus tessae n. sp. is descriptionbed from the Fish River in South West Africa/Namibia. The new species is characterized by a slender, cylindroid body, outer labial setae slightly shorter than lip region diameter, amphideal fovea one third corresponding body diameter, elongate-conoid tail equal to about three fourths the vulva-anus distance, absence of posterior uterine sac. The spicules are equal, slightly bent and barely longer than anal body diameter. T. tessae n.sp. is compared with the cosmopolitan Theristus agilis (de Man, 1880) as well as with all other known athallasic species. Keywords: Nematoda: Xyalidae, Theristus, Theristus tessae.