Red grouper (Epinephelus akaara) females between two and four years of age were successfully reversed to functional males by feeding 17 α -methyltestosterone (17 α -MT) over 42 days. A gene, called ECaM, was cloned from sex-reversed male gonads by using the combinative methods of suppressive subtraction hybridization (SSH), SMART cDNA synthesis and RACE-PCR. The full-length cDNA of ECaM is 582 bp, containing a 450 bp open reading frame that encodes a 149 amino acid protein. It has a 5 ′ untranslated region (UTR) of 74 nt and a 3 ′ UTR of 58 nt. Virtual Northern blotting shows that ECaM is transcribed in sex-reversed male gonads but only slightly transcribed in normal female gonads. Semi-quantitative RT-PCR analyses from various tissues indicate that mRNA of ECaM can be detected in the brain, heart, liver, spleen, and kidney but slightly in the muscle. Semi-quantitative RT-PCR and Western blotting from gonads during various stages of sex reversal reveal that expression levels of ECaM in gonads increase gradually during the transformation from female to male. Calmodulin is highly conserved across species. ECaM is one of the important genes that impels grouper sex reversal.