Effects of external cation concentrations on larval metamorphosis in the ascidian, Halocynthia roretzi were examined. Metamorphosis (tail resorption) was induced by 20 mM and 50 mM K+ but, interestingly, was suppressed by 30 mM K+. Low concentrations of Ca2+ (1–5 mM) induced metamorphosis, while high concentrations did not, in contrast to findings in other invertebrates. Moreover, BAPTA-AM, an intracellular Ca2+ chelator, induced metamorphosis. This suggests that a decrease in intracellular Ca2+ of certain cells initiates larval metamorphosis in H. roretzi.