Specific FHFs (Fibroblast growth factor homologous factors) regulate voltage-gated sodium channel (VGSC) function by modulating activation, inactivation and/or subcellular distribution through binding to the C-terminus (CT) of specific VGSC alpha subunits. The effect upon VGSC function varies greatly depending upon the particular FHF isoform and its VGSC binding partner, but the specificity of these pair wise interactions are not understood. To identify the determinants for interaction between specific FHFs and VGSC pairs, we studied binding in a recombinant bacterial co-expression system. We show that FHF1b (aa 1- 144), which contains the conserved FHF β-trefoil core domain and is 60-70% homologous to the other members of the FHF family, bound to a NaV1.5 CT containing aa 1773-1940, but not to a NaV1.5 CT comprised of aa 1773-1878. Since the part of the CT necessary for binding contains the calmodulin (CaM) interaction site, we tested whether CaM and FHF1b competed for binding and found that all three components could form a complex; thus, CaM and FHF do not compete for interaction with the NaV1.5 CT. A full length FHF1b and FHF2b also bound to the NaV1.5 CT (aa 1773-1940) but not NaV1.5 CT (aa 1773-1878), suggesting that the region between aa 1878-1940 is necessary for FHF interaction. FHF1b also bound to NaV1.6 (aa 1767- 1926), but did not bind NaV1.1 (aa 1787-1948) nor NaV1.2 (aa 1772-1937). Since the distal regions among the tested NaV1.x CTs are less similar than the proximal regions, we predict that the specificity for interaction and the consequent particular effects upon VGSC function derives at least in part from this less-well conserved NaV1.x region.