The zebrafish (Danio rerio) has been developed as a powerful model for genetic studies in developmental biology, which give insights into several human diseases such as cardiovascular disease and cancer. Because aging processes affect these and many other human disorders, it is important to compare zebrafish and mammalian senescence. However, the aging process of zebrafish remains largely unexplored, and little is known about functional aging and senescence in zebrafish. In a survey of aging zebrafish, we detected senescence-associated b-galactosidase activity in skin and oxidized protein accumulation in muscle. However, we did not observe lipofuscin granules (aging pigments), which accumulate in postmitotic cells, in muscle of zebrafish with advancing age. This may be consistent with the existence of continuously proliferating myocytes that incorporated BrdU in muscle tissues of aged zebrafish. Moreover, we demonstrated that zebrafish have constitutively abundant telomerase activity in somatic tissues from embryos to aged adults implicating unlimited replicative ability throughout their lives. Although some stress-associated markers are upregulated and minor histological changes are observed during the aging process of zebrafish, our studies together with other evidence of remarkable reproductive and regenerative abilities suggest that zebrafish show very gradual or sub-negligible senescence in vivo.