Abstract: This study reported a new monogenean species, Ancyrocephalus carassii sp. Nov, isolated from gills of Carassius auratus purchased in the Weigang Market, Nanjing, China. Its body length and width are 1.12 mm and 0.32 mm, respectively. Opisthohaptor consists of 7 pairs of embryonic marginal hooklets, 2 pairs of anchors and 2 connective plates. Total length of dorsal anchor, shaft, point, and inner root are 0.073 mm, 0.056 mm, 0.028 mm, and 0.035 mm, respectively, and outer root is not obvious. Total length of ventral anchor, shaft, point, inner root, and outer root are 0.059 mm, 0.047 mm, 0.02 mm, 0.03 mm, and 0.012 mm, respectively. The size of dorsal connective plate and ventral connective plate are 0.015 mm×0.1 mm and 0.014 mm×0.078 mm, respectively. Total length of marginal hooklet is 0.025 mm, with its point being 0.007 mm and the shaft being 0.016 mm. Thin-long copulatory tube winds up. It is about 0.256 mm in length while it’s bulb-like root is 0.012 mm in diameter, and its supporting apparatus is about 0.033 mm in length. Vagina coiling is 0.119 mm in length, and its trumpet-like opening that connects with supporting apparatus is about 0.45 mm. The chitinous structure of this species is pretty similar to those of A. mogurndae on Siniperca chuatsi, suggesting a relative relationship between the two species of Ancyrocephalus. This is the first report of ancyrocephalid on C. auratus.