A telomerase reverse transcriptase (Tert) encoding gene was cloned from the testis of the teleost fish Oryzias latipes. The expression pattern of Japanese medaka tert (Ola_tert) was analyzed by reverse transcription–polymerase chain reaction and in situ hybridization. Ola_tert was expressed in embryonic stages as well as in differentiated adult tissues. In tissues of adult medakas the highest tert expression was found in gonads and brain. Furthermore, two different splice variants were described and an Ola_tert antisense transcript was identified. The enzyme activity of Tert was determined using a non-radioactive telomeric amplification protocol and the telomerase activity in various tissues was shown to correlate with the tert expression. The telomerase activity was found to be high in contrast to the generally low activity in differentiated human tissues.