Black porgy, Acanthopagrus schlegeli Bleeker, is a marine protandrous hermaphrodite fish. All are functional males at 1–2 yr of age and then become either males or females at 3 yr of age. To study the process of sex change in this species, mRNA transcripts of two estrogen receptors (ERα and ERβ) and an androgen receptor (AR) were monitored. An AR cDNA was cloned and characterized. ERα, ERβ, and AR were differentially transcribed in bisexual testicular and ovarian tissue according to reverse transcription polymerase chain reaction (RT-PCR) and Southern analysis. A real-time quantification PCR analysis was further developed for the measurement of AR, ERα, and ERβ transcripts. ERα and AR transcripts were significantly more plentiful in bisexual testis than in bisexual ovary in 1 - and 2 -yr-old fish. ERα, ERβ, and AR transcripts decreased in the functional testis of 3-yr-old fish. Similar levels of ERβ and AR were detected in the ovary of sex-changed females and in functional testis of 3-yr-old males. Significantly decreased AR transcripts were found in testicular tissue of bisexual and functional male and female gonads in 3-yr-old fish as compared with 1- and 2-yr-old fish. In contrast, increased ERα transcripts were detected in the bisexual ovary and sex-changed ovary of 3-yr-old fish as compared with the bisexual ovary of 1- and 2-yr-old fish. The data suggest a differential sensitivity in the bisexual testicular and ovarian tissue of black porgy.