Contraction of cardiac muscle is triggered by an intracellular buildup of Ca²⁺ during excitation‐contraction (E‐C) coupling. The Na⁺/Ca²⁺ exchanger (Ncx1) is highly expressed in cardiomyocytes and is thought to serve a housekeeping function by maintaining a low intracellular Ca²⁺ concentration. However, its role in E‐C coupling is controversial. To determine the precise role of Na⁺/Ca²⁺ exchange in development of the mammalian heart, we used gene targeting to delete Ncx1. Heterozygous mice are normal and fertile, whereas Ncx1‐null embryos are growth‐retarded and survive to 11.0 days postcoitum but lack a spontaneously beating heart. Moreover, normal heart morphogenesis (specification, looping, and chamber formation) occurred relatively normally within Ncx1‐null embryos. In addition, Ncx1‐nulls displayed relatively normal transient Ca²⁺ signals when electrically stimulated. This suggests that the Ca²⁺ delivery mechanism was fundamentally intact, and that Ncx1‐null cardiomyocytes can regulate intracellular Ca²⁺ concentrations despite the absence of Ncx1. However, ultrastructural analysis revealed that Ncx1‐null cardiomyocytes have a complete lack of organized myofibrils and Z‐lines when compared with normal littermates. These data demonstrate that Ncx‐1 is a Ca²⁺‐ gene that is essential for normal cardiomyocyte development and function and may serve as an animal model for functionally related human congenital heart defects.