Striated muscle cells display an extremely regular assembly of their actin cytoskeleton that contributes to the contractile elements, the myofibrils. How this assembly is initiated and how these structures are maintained is still unclear. We have recently shown that striated muscle expresses a specific isoform of the formin protein family member FHOD3, which is characterised by the presence of a CK2 phosphorylation site at the C-terminal end of the formin homology domain 2 (FH2). Phosphorylated muscle FHOD3 displays a different subcellular localisation, namely to the myofibrils, and also has increased stability compared to un-phosphorylated or non muscle FHOD3. In addition, we could show that muscle FHOD3 is involved in myofibril maintenance in cultured cardiomyocytes and that its presence dramatically enhances the reconstitution of cardiac actin filaments after depolymerisation. Since FHOD3 expression levels and in particular that of the muscle isoform are also decreased in different types of cardiomyopathy, we postulate a crucial role for this protein in the maintenance of a fully functional cardiac cytoarchitecture.