The objectives of this study were to investigate the presence and annual cycle of sex steroids in scleractinian coral, Euphyllia ancora. The free and conjugated forms of sex steroids in coral and spawning seawater were investigated, and aromatase activity in the coral tissue was identified. Polyps collected from corals and seawater were extracted with diethyl ether, and purified by alumina column and reversed-phase HPLC; testosterone and estradiol-17β (E₂) was measured by a validated RIA. E₂ and testosterone in their free and glucuronide forms were consistently detected in coral tissue throughout the year. Peak concentrations of free E₂, E₂ glucuronide, and testosterone glucuronide were obtained in the coral tissue just prior to spawning. The presence of specific aromatase activity was demonstrated in coral tissue. Free E₂ and E₂ glucuronide concentrations were higher than androgen (testosterone and testosterone glucuronide) in coral tissue and spawning seawater. Higher concentrations of free E₂ than E₂ glucuronide were detected in coral tissues throughout the year. In contrast, higher concentrations of E₂ glucuronide than free E₂ and testosterone glucuronide were found in seawater during mass coral spawning. No steroid sulfate could be detected in the coral tissue and seawater. We suggest that the release of E₂ glucuronide may play an important role in coral mass spawning.