In samples from Mekong River Delta 69 species of free-living nematodes were found, 23 Óf them were new for science species. Saline-water and marine nematode forms prevailed. The dominant species were Daptonema iners, Terschellingia longicauda and Paracomesoma curvitatus. A list of nematode species, the description and illustrations of Longicyatholaimus subtenuis sp.n. as new for the science species are presented. A new species resembles L. longicaudatus (de Man, 1876) and L. tenuicaudatus (Saveljev, 1912), but differs from the former in a shorter body, tail and spicules, anothes structure of gubernaculum, absence of precloacal supplements, cervical and somatic setae. From L. tenuicaudatus it differs in the longer cephalic setae, shorter spicules and another structure of gubernaculum.