Dagger nematodes ( Xiphinema spp.) are a group of ectoparasitic nematodes that cause damage to an extensive range of crops by their feeding on plant root cells and the important is that some of which are virus vectors and can transmit viruses to many economically important crops. Many countries and organizations in the world have listed them as quarantine pests. Due to the morphological overlapping of the key identification characters among the species, it is very difficult only based on the morphological characters to identify the species. In this study, Xiphinema populations from 14 proviences of China were systemitical studied based on the morphological and rDNA characters. The major results obtained are as follows:1. Among 1,599 soil samples collected from 14 provinces of China, including Beijing, Shandong, Shanxi, Hebei, Henan, Jingsu, Anhui, Sichuan, Zhejiang, Fujian, Yunnan, Hainan, and Guangdong, 19.9% of which were detected the presence of Xiphinema populations, and 192, 63 and 41 soil samples contain X. americanum group species, X. insigne and X. hunaniense, respectively. The three species account for 58%, 19% and 12% of the total Xiphinema populations detected and this shows they are predominant species in the genus Xiphinema occurring in China. All the Xiphinema populations studied in this research have four juvenile development stages in their life cycles.2. Based on the morphological and molecular characters, the Xiphinema americanum group populations occurring in China used in this study were grouped and identified into three species, X. brevicollum, X. incognitum and X. oxycaudatum, in which X. oxycaudatum was reported for the first time in Fujian. According to morphological characters, ITS regions of rDNA and D2D3 regions of 28S RNA gene, we found that X. brevicollum is widely distributed in China. Molecular data support that X. incognitum is a valid species, and the validity of X. diffusum should be questioned. A standard rDNA PCR-RFLP protocol to differentiate and identify the X. americanum group species occurring in China was constructed.3. Morphometrical comparisons revealed great variation between intra-species populations of X. insigne from different geographic regions of China (Hainan, Shandong, Sichuan, Zhejiang). All the major morphometrics, length of odentostyle, length of tail and V value are overlapping, and have no relativity with the species and populations. The results disapproved the presence of two forms of X. insigne, indicum-form and insigne-form. Sequence alignments of ITS region of rDNA and D2D3 region of 28S RNA gene revealed the sequence consistency between these populations. Here we report for the first time, the distribution of X. insigne in Hainan province and first stage juvenile in China.4. Information about the D2D3 expansion region of 28S RNA gene and first stage juveniles of X. hunaniense were reported for the first time. Molecular data support X. hunaniense as a valid species and a morphologically special X. hunaniense population was described. Morphological and molecular variations among intra-species of X. hunaniense were provided and compared.5. Morphological and molecular informations of X. elongatum parasiting on roots of sugarcane in Hainan province were repored, and this is the first report about the distribution of X. elongatum in Hainan province.6. Based on both morphological and molecular characters, two new Xiphinema species were described and reported. Xiphinema n. sp.1 is from the rhizosphere of an unknown arbor in Shenyang, Liaoning. The identification code in the polytomous key of Xiphinema (Luc & Loof, 1990) for the species is A4-B4-C4-D(4)5-E3-F3(4)-G(2)3-H2-I3-J3-K?-L1. The rDNA sequence has a relatively high simility to X. bakeri but differs from it in body length and V value. Xiphinema n.sp.2 collected from the rhizosphere of Fagaceae in Hangzhou, Zhejiang and morphologically resembles X. chambersi but differs from it in rDNA sequences (The indentity percent of ITS1 region of rDNA of two species is only 85.7%). The polytomous code of Xiphinema n.sp.2 is A1-B4-C2-D3-E1-F2-G2-H2-I3-J2-K?-L1. Morphological description of a X. brasiliense population collected from Xiamen, Fujian province was provided.7. Phylogenetic trees of major Xiphinema species and populations from China used in this study and related species were constructed based on sequences of the ITS region of rDNA and D2D3 region of 28S RNA gene.The mam results obtained in this study will be helpful in constructing standard system for morphological and molecular taxonomy and identification of Xiphinema species, and it will be very useful on taxonomy and identification of Xiphinema species occurring in China.