Cardiac myosin-binding protein C (cMyBP-C), a major accessory protein of cardiac thick filaments, is thought to play a key role in the regulation of myocardial contraction. Although current models for the function of the protein focus on its binding to myosin S2, other evidence suggests that it may also bind to F-actin. We have previously shown that the N-terminal fragment C0-C2 of cardiac myosin-binding protein-C (cMyBP-C) bundles actin, providing evidence for interaction of cMyBP-C and actin. In this paper we directly examined the interaction between C0-C2 and F-actin at physiological ionic strength and pH by negative staining and electron microscopy. We incubated C0-C2 (5-30μM, in a buffer containing in mM: 180 KCl, 1 MgCl(2), 1 EDTA, 1 DTT, 20 imidazole, at pH 7.4) with F-actin (5μM) for 30min and examined negatively-stained samples of the solution by electron microscopy (EM). Examination of EM images revealed that C0-C2 bound to F-actin to form long helically-ordered complexes. Fourier transforms indicated that C0-C2 binds with the helical periodicity of actin with strong 1st and 6th layer lines. The results provide direct evidence that the N-terminus of cMyBP-C can bind to F-actin in a periodic complex. This interaction of cMyBP-C with F-actin supports the possibility that binding of cMyBP-C to F-actin may play a role in the regulation of cardiac contraction.