The present study demonstrated here for the first time that there are statistically significant differences in sterol composition between male and female gonads of the dominant limpets Cellana grata and Cellana toreuma, which are intertidal gastropods. Among 11 different sterols identified in this study, unusually high levels (11.2-19.8% of total sterols) of the Delta8-sterols 5alpha-cholest-8-en-3beta-ol (zymostenol) and 5alpha-cholesta-8,24-dien-3beta-ol (zymosterol), which have never been reported in aquatic invertebrate gonads, were present in only the male gonads.