Although the adult mammalian heart is incapable of meaningful functional recovery following substantial cardiomyocyte loss, it is now clear that modest cardiomyocyte turnover occurs in adult mouse and human hearts(1,2), mediated primarily by proliferation of pre-existing cardiomyocytes(3-5). However, fate mapping of these cycling cardiomyocytes has not been possible thus far owing to the lack of identifiable genetic markers(6). In several organs, stem or progenitor cells reside in relatively hypoxic microenvironments where the stabilization of the hypoxia-inducible factor 1 alpha (Hif-1 alpha) subunit is critical for their maintenance and function(7-10). Here we report fate mapping of hypoxic cells and their progenies by generating a transgenic mouse expressing a chimaeric protein in which the oxygen-dependent degradation (ODD) domain of Hif-1 alpha is fused to the tamoxifen-inducible CreERT2 recombinase. In mice bearing the creERT2-ODD transgene driven by either the ubiquitous CAG promoter or the cardiomyocyte-specific a myosin heavy chain promoter, we identify a rare population of hypoxic cardiomyocytes that display characteristics of proliferative neonatal cardiomyocytes, such as smaller size, mononucleation and lower oxidative DNA damage. Notably, these hypoxic cardiomyocytes contributed widely to new cardiomyocyte formation in the adult heart. These results indicate that hypoxia signalling is an important hallmark of cycling cardiomyocytes, and suggest that hypoxia fate mapping can be a powerful tool for identifying cycling cells in adult mammals.