Sex change in the coral-dwelling goby Gobiodon histriowas induced by placing two adult fish of the same sex on a coral colony. The sex change of individual fish was confirmed using histology, and whole-body concentrations of the gonadal steroids testosterone (T), 11-ketotestosterone (11-KT), and17b-oestradiol (E2)were examined.The results showthat T,11-KTandE2 occurred inboth female and male G. histrio. E2 concentration in females was twice that in males, while concentrations of T did not differ between the sexes. Contrary to predictions, concentrations of Tand E2 did not differ between fish that changed sex and those that did not. Most samples had 11-KT concentrations belowminimumlevels of detection (i.e.<0 15 ng ml 1) andwere therefore not analysed statistically. The results suggest that: (i) specific activation or de-activation of theT^E2 (aromatase) pathway is a probable candidate for mediating serial adult sex change in G. histrio, and (ii) low levels of 11-KT may be important in allowing serial adult sex change in G. histrio.