To better improve understanding of the molecular mechanisms involved in the actions of gonadotropins in fish, a structural and functional characterization of trout gonadotropin receptors has been undertaken. The present study describes two distinct gonadotropin receptors in trout showing similarities with those of other vertebrates but also differences in structural determinants of the FSHR. As observed in their mammalian counterparts, trout LH receptor (LHR) harbours a large N-terminal hormone binding domain with 9 leucine reach repeats (LRR) flanked by a proximal and distal cysteine rich region (CRR). A similar sequential organization of 9 clustered LRR and 2 CRR motifs is also observed on mammalian FSH receptors however this is absent in the trout with only one out of four conserved cysteine residues being present upstream of the first LRR motif. The functional specificity of trout gonadotropin receptors was investigated in the heterologous mammalian COS-7 cells using homologous (trout) and heterologous (chinook salmon) purified gonadotro- pins. Contrary to published data in catfish and zebrafish, trout gonadotropin receptors show a high selectivity for their respective cognate ligand.