Tilapia GtHII cDNA was amplified and isolated using RACE PCR on total RNA from a single tilapia (Oreochromis niloticus × O. aureus) female. The cDNA (from the first amino acid of the mature peptide) was cloned, sequenced and found to be 480 bp. Comparison of the deduced amino acid sequence of tilapia with that of GtHII from other teleosts revealed that it shares 67% homology with the sequence of the chum salmon (Oncorhynchus keta) and 96% to the grouper (Epinephelus aeneus). The genes coding for the subunits of GtHI and II were isolated from a tilapia genomic library. The GtHI clone was identified using the gilthead seabream (Sparas aurata) GtHI cDNA sequence as a probe. The deduced amino acid sequence obtained showed that it is 56% homologous to that of the gilthead seabream and 60% homologous to that of the bonito (Katsuwonus plelamis). The gene coding for the GtHII subunit was isolated using the tilapia GtHII cDNA probe. Both genes contain a large intron at the conserved position which is three amino acids downstream from the fifth cystein. Northern blot analysis revealed that in immature tilapia males both genes are expressed: GtHI and GtHII transcripts are about 500 and 600 bases long, respectively.