The new species Iotonchus lotilabiatus n. sp. from Bat Xat Nature reserve in Lao Cai Province, Vietnam is described and the molecular data (18S and 28S rDNA) are given. Females of the new species are characterized by large body size (L = 3.8-5.2 mm); barrel shaped buccal cavity of large size (41–54×70–89 µm) with dorsal tooth apex located 22 to 23.5% of buccal cavity length from its base; reproductive system didelphic-amphidelphic, vagina long, corresponding almost half of body width at the vulva with distinct parrefringens vaginae, sclerotized and triangulated pieces in optical section; tail long filiform, ventrally arcuate, with three small caudal glands in tandem and prominent terminal spinneret. Males with spicules 119 to 148 µm in length, and slender, slightly curved gubernaculum, 39 to 44 µm long and with 13 to 14 ventromedian supplements. The new species Iotonchus lotilabiatus n. sp. is closest to I. miamaensis (Khan and Araki, 2002), but differs by having larger size of body length and buccal cavity, lower position of dorsal tooth apex; longer female tail with lower c but higher c′ ratios and the presence of advulval cuticular structures.
