Recent data showing the occurrence of IGF in fish, mainly teleostean fish, and their biological activity in gonads are reviewed. IGF-I does occur in all vertebrates and first evidence of IGF-II occurrence in fish has been reported. IGF-I is expressed in the testis and the ovary, and receptors for the peptide have been demonstrated in both gonads. Besides, IGF-I seems implied in spermatogonial proliferation and in oocyte maturation. Both germinal cells and somatic cells could be responsive to IGF-I stimulation.