Abstract: Chydorus undulatus Chiang, 1963 Female: Length 0.34—0.51mm. Body broadly oval, with brownish to golden yellow tinge. Shell reticulated, with 2—4 rows of crescentic sculptures along the ventral border, variable in its development. Dorsal margin strongly arched, with posterior margin sloping steeply. Ventral margin curved. Head small, with crescentic sculptures also. Rostrum long and sharp, keeled below and closely near the anterior margin of the shell. Labrum beaked, labral keel smooth. Antennules short, not reaching tip of rostrum, one olfactory seta situated laterally, the others terminal. Antennae small, last segment of each branch with a small spine at the tip, antennal formula 0-0-3/0-1-3. Postabdomen short, dorsal margin armed with 8 or 9 denticles of about the same length, lateral fascicles present. Postabdominal claw large, with two basal spines, distal one small, proximal one quite short and slender.Male: Unknown.Distribution: Qinghai and Sichuan Province. China.