320 ISSN 1995-0829, Inland Water Biology, 2008, Vol. 1, No. 4, pp. 320–325. © Pleiades Publishing, Ltd., 2008. Original Russian Text © V.G. Gagarin, Nguyen Thi Thu, 2008, published in Biologiya Vnutrennikh Vod, No. 4, 2008, pp. 12–17. INTRODUCTION The fauna of free-living nematodes in Vietnamese waterbodies has not been studied until recently. Only in the 21st century have intense studies of this group of worms begun as part of the work on the data bank of aquatic fauna in major waterbodies and water courses of Vietnam (Gagarin and Nguen Vu Thanh, 2004, 2005, 2006, 2006a, 2007; Gagarin and Nguen Thi Thu, 2008). The purpose of this study is to reveal the species composition of the fauna of free-living nematodes of the Red River Delta. MATERIALS AND METHODS This study is based on 19 samples of nematodes col- lected in April 2004 by specialists from the Institute of Ecology and Biological Resources of the Vietnamese Academy of Science and Technology (Hanoi, Vietnam) in the Red River (also known as the Hoang) delta in the Txai Binh Province, Bat Lat Region, in northern Viet- nam. This is the largest river of this territory and holds the most water. The samples were taken 1–3 m from the bank at a depth of 1.0–2.5 m from the silty bottom. The higher plants in this area were represented by dense mangroves; the salinity of the water was 8.1–28.6‰. Samples were taken from a boat with a tubular bottom sampler and sifted through a bolter with a mesh size of 80 μm, before 4% formaldehyde was added to the pre- cipitate. The worms were identified and measured using a MBB-1 microscope and glycerol preparations. Eight new nematode species were found in the sam- ples. One of these species is described in this paper. The following abbreviations are used in the description: L (body length); a (ration of body length and maximum width); b (ratio of body length and oesophagus length); c (ratio of body length and tail length); c' (ratio of tail length and diameter of body near anus (cloaca)); V (ratio of distance between anterior apex of body and vulva, and total body length, %). Preparations of the holotypes and some of the paratypes of the new species are stored in the collection of the Institute for Biology of Inland Waters, Russian Academy of Sciences, Borok, Yaroslavskaya oblast’, Russia. Most of the paratype preparations are stored in the collection of Nematology Department, Institute of Ecology and Biological Resources, Hanoi, Vietnam. RESULTS Fauna. Fifty-four nematode species were found in the samples, eight of which are new to science. Four- teen nematode species were not identified to a species because they are represented by few individuals (1–3) of the same sex. The identified species belong to eight orders. The order represented by the largest number of species is Monhysterida (18) and the order represented by the smallest number is Dorylaimida (3) (see Table 1). Most of the species are brackish-water and marine forms of nematodes. This is probably due to the high salinity of water in the area of sample collecting and by the proximity of the sea. Four species (Laimydorus oxu- rus, Aquatides thornei, Monhystera sp., and Eumonhys- tera sp.) belong to fresh-water forms. Longidorus sp. is considered a plant parasite and was probably found in the water course by chance, having been washed into the river from the bank. Brackish-water species Sphaerotheristus validum, Terschellingia lissa, Dap- tonema sp., and Sabatieria sp. are represented in the samples by the largest number of individuals. Description of the species Viscosia timmi sp. n. (see the figure and Table 2). Free-Living Nematodes from the Red River Delta, Vietnam V. G. Gagarin a and Nguyen Thi Thu b a Papanin Institute for Biology of Inland Waters, Russian Academy of Sciences, Borok, 152742 Russia e-mail: gagarin@ibiw.yaroslavl.ru b Department of Nematology, Institute of Ecology and Biological Resources, Vietnamese Academy of Science and Technology, 15 Hoang Quoc Viet Rd., Hanoi, 10000 Vietnam Received May 27, 2007 Abstract—In samples from the Red River delta in northern Vietnam, 54 species of free-living nematodes were found, 8 of which are a species new to science. Sphaerotheristus validum, Terschellingia lissa, Sabatieria sp., and Daptonema sp. predominated. A checklist of nematode species is given, as is an illustrated description of Viscosia timmi sp. n. DOI: 10.1134/S1995082908040020 AQUATIC FLORA AND FAUNA