Amphibian embryos exposed to water of low pH were killed by two distinct mechanisms. At very low pH levels, embryos stopped development soon after exposure to test solutions. At higher but still lethal pH levels, embryos became curled within a shrunken perivitelline space and failed to hatch (curling defect). The addition of Ca, Mg, and to a lesser extent Na (greater than 10 mg/l), prevented the early mortality of embryos in acidic water. However, increasing concentrations of these ions also caused the curling defect. Embryos of Ambystoma maculatum and Ambystoma jeffersonianum were generally able to hatch even though they became curled, but Rana sylvatica remained trapped and died. Consequently, as the concentration of Ca, Mg, or Na was increased at low pH, greater numbers of embryos of A. maculatum and A. jeffersonianum hatched, while survival of embryos of R. sylvatica was drastically reduced.