Abstract: An acute toxicity testing of Cu²⁺ exposure on the Gobiocypris rarus embryos was conducted in which the G. rarus embryos were exposed to Cu²⁺ solutions at different concentrations of 0.01, 0.05, 0.1, 0.2 mg·L^-1 , so as to see its effect on the early development of a fertilized egg. The findings showed that Cu²⁺ had significant toxic effects on G. rarus embryos. The major toxic effects were found to be the following: an increased mortality rate, a slowing-down heart rate and arrhythmia; a decrease of the newborn larval fish in its body length and the occurrence of pericardium and yolk sac edema; and all embryos died at high concentration of Cu²⁺ (above 0.1 mg·L^-1)after exposure of 60h.