Eukaryotic chromosome termini consist of telomeres, short sequence repeats. According to the telomere hypothesis, DNA replication leads to telomere shortening, resulting in a cellular mitotic clock. Telomerase resets it by telomere synthesis. In mammals with a limited growth phase, telomerase activity in somatic tissues is restricted to stem cell derivatives with high proliferation potential. But other animals, like some fish, grow throughout their life with little senescence. All somatic cells require a high proliferation capacity and telomerase should be active in all cells, irrespective of fish age. Indeed, we detected high telomerase activities in all analyzed organs of rainbow trout (Oncorhynchus mykiss).