A clear understanding of myocardial development is essential not only for understanding of the molecular basis of congenital heart disease and its prevention but also for successful regeneration after cardiac injury. A recent study employed a novel Cre/LoxP-based lineage tracing approach with a multi-color reporter in zebrafish to examine the fates of populations of developing cardiomyocytes. The results showed that a remarkably few number of clones of cardiomyocytes are involved in the formation of adult zebrafish heart. Furthermore, a striking difference in the mechanism of myocardial compaction was described involving the creation of a completely new layer of cortical myocardium.