Using immunofluorescence histochemistry and immunoelectron microscopy on sections through myocardiac tissues of diverse mammalian (human, cow, rat, mouse) and fish species we show that both desmosomal and fascia adhaerens proteins identified by gel electrophoresis and immunoblot occur in the area composita, the by far major type of plaque-bearing junctions of the intercalated disks (IDs) connecting cardiomyocytes. Specifically, we demonstrate that desmoplakin and the other desmosomal proteins occur in these junctions, together with N-cadherin, cadherin-11, alpha- and beta-catenin as well as vinculin, afadin and proteins p120(ctn), ARVCF, p0071, and ZO-1, suggestive of colocalization. We conclude that the predominant type of adhering junction present in IDs is a junction sui generis, termed area composita, that is characterized by an unusually high molecular complexity and an intimate association of molecules of both ensembles, the desmosomal one and the fascia adhaerens category. We discuss possible myocardium-specific, complex-forming interactions between members of the two ensembles and the relevance of our findings for the formation and functioning of the heart and for the understanding of hereditary and other cardiomyopathies. We further propose to use this highly characteristic area composita ensemble of molecules as cardiomyocyte markers for the monitoring of cardiomyogenesis, cardiomyocyte regeneration and possible cardiomyocyte differentiation from mesenchymal stem cells.