Abstract:In the present paper a new species, Bathythrix cilifacialis Sheng parasitizing in the cocoon of Pachynematus itoi Okutani (Hymenoptera: Tenthredinidae), with its biology is described and illustrated.Type specimens are deposited in the General Station of Forest Pest Management, Chinese Ministry of Forestry.Bathythrix cilifacialis Sheng, sp.nov. (Fig.)♀♂: Body 5.1～7.0 mm long. Fore wing 4.0～5.5 mm long. Face with very dense and long silver hairs, which are downward and press close to its surface. Clypeus with two strong teeth at apex. Malar space about 0.4 time (♀) and 0.3 time (♂) as long as basal width of mandible. Oral carina strongly raised behind the base of mandible. Antenna with 26～27 flagellar segments, in ♂ without tyloids. Basal vein of fore wing nearly parallels discocubitus. Nervellus distinctly reclivous, intercepted at lower 0.3. First segment of abdomen with almost parallel sides, 3.6～3.9 times as long as its apical width, spiracles at the midlle, without median dorsal carina. Second tergite a little longer than its apical width. Ovipositor straight. Ovipositor sheath 1.20～1.25 times as long as hind tibia. Black. front and midlle legs mostly yellow to yellowish brown. Tergite 2～4 redish brown. Holotype♀, Benxi (41°N, 123.8°E), Liaoning, Aug. 16, 1997. Sheng Maoling. Allotype♂, Aug. 20, 1997. Other data as the holotype. Paratypes: 2♀♀, Sep. 17～22, 1997. Other data as the holotype. This species is similar to Bathythrix. tenuis (Gravenhorst, 1829), but may be distinguished from the latter by: temple with shallow sparse punctures; antenna with 26～27 flagellar segments, ovipositor straight; tergite 2～4 redish brown. Biology: Adults occur in August～September, take cover beneath the fallenleaves, favor to drink water on the emergence day. It parasitize larva and prepupa in cocoon of Pachynematus itoi Okutani.