Klotho is an aging suppressor gene. In mice, loss of Klotho function causes accelerated aging while increased Klotho expression increases longevity. This study aimed to identify and characterize the orthologue of Klotho in zebrafish, a powerful model organism for the investigation of development and human disease. Zebrafish klotho was identified by a bioinformatics approach, and cloning and sequencing of klotho cDNA confirmed the in silico analysis. The zebrafish Klotho protein has a structure similar to human and mouse Klotho, but it lacks an apparent secretory signal sequence. We can find no evidence of an alternative transcript isoform lacking the transmembrane domain coding sequence as seen in mammals. RT-PCR revealed the expression of klotho during embryonic development and in a wider variety of adult tissues than in mouse. Quantitative real-time RT-PCR demonstrated the relative gene expression profile of zebrafish Klotho during embryogenesis and in adult tissues. In situ hybridization showed an apparently diffuse signal of klotho mRNA expression in the adult zebrafish testis. ; Yuya Sugano, Michael Lardelli