Summary The toxicity of copper sulphate (1 mg/L) during embryonic development of the shrimp Palaemonetes pugio was tested. One group of embryos was exposed to copper during their entire period of development, a second during the first half of the developmental period and a third group during the second half. A fourth group was maintained in clean water throughout, serving as control. Eggs were checked daily to determine embryo mortality and the incidence of hatching. The experimental group continuously exposed to copper, as well as that exposed during the second half of development, resulted in higher embryo mortality with a consequent decreased number of hatched larvae when compared to the control group. Therefore, the later period of embryogenesis was more sensitive to copper than the early one. Deformed eyespots in embryos were seen in all groups exposed to copper, while underdeveloped setae were noted in larvae hatched from eggs exposed to copper either during the entire or only the second half of development period.