(Z)- and (E)-narains, 3,4-dihydroxystyryl sulfates, have been isolated from a marine spongeJaspissp. as inducers of larval metamorphosis in the ascidianHalocynthia roretzi(Tsukamoto et al., 1994a, 1994b). Subsequently, we found that narains inhibited hatching inH. roretziat concentrations of 10–50 μM, but did not affect trypsin-like protease activity which is released during larval hatching. Isolated chorion was dissolved by crude hatching enzyme solution and by commercially available trypsin from bovine pancreas. In both cases, dissolution of the chorion was inhibited by narains. Furthermore, the follicle cells surrounding the chorion of narain-treated embryos became condensed. These results suggest that narains do not inhibit hatching enzyme activity but rather affect the envelope of the embryo, resulting in inhibition of hatching.