Abstract: Butachlor is one of the most widely used amide herbicides. Although the researches on the reproductive toxicity of butachlor to freshwaterfish have been reported extensively, few studies addressing the reproductive toxicity of butachlor on the offshore fishes are available. The present study was conducted to explore the effects of butachlor on ovarian development inSebastiscus marmoratus, a common offshore fish distributes in the southeast coast of China, and to gain insight into its mechanism. After exposing to butachlor at environmental concentrations (2, 20 and 200 ng/L) for 50 d, the development of ovarian germ cell was depressed to different extents. While 17β estradiol (E2) was decreased in the ovaries, the activity of Caspase-3 increased. Realtime PCR showed that the expressions of CYP19s and ERβ were down-regulatedin the ovaries. These results indicated that butachlor can cause significant reproductive toxicity to Sebastiscus marmoratus, and the inhibited expression of CYP19s gene and ERβ in the ovary leads to the decrease of estrogen level and apoptosis of egg cells in early development.