Abstract: The common mineral floating agent, butyl xanthate has been studied with reg-ard to its toxic effects on the early development of grass carp, Ctenopharyngodon idellus. When exposed to lethal and sublethal concentrations of butyl xanthate,adverse responses were observed in hatching, surviving, growth and teratogeny of the embryos.The 96-hours median lethal concentration(LC50)for embryos was 17.8mg/l.The median teratogenic effective concentration(EC50)was 8.85mg/l. Teratogeny was induced at the concentration of 5.6mg/l.Furthermore,adverse effects on growth were observed at the concentration of 5.6-10mg/l.Hatchability was only 38.8% in 24mg/l.The butyl xanthate can therefore be considered being teretogenicto embryos of grass carp.Main teratological symptoms of the embryo were kyphosisand neoplasm on the body surface.