In this study we demonstrate a direct interaction between a cytosolic fibroblast growth factor family member and a sodium channel. A yeast two-hybrid screen for proteins that associate with the cytoplasmic domains of the tetrodotoxin-resistant sodium channel rNa(v)1.9a (NaN) led to the identification of fibroblast growth factor homologous factor 1B (FHF1B), a member of the fibroblast growth factor family, as an interacting partner of rNa(v)1.9a. FHF1B selectively interacts with the C-terminal region but not the other four intracellular segments of rNa(v)1.9a. FHF1B binds directly to the C-terminal polypeptide of rNa(v)1.9a both in vitro and in mammalian cell lines. The N-terminal 5-77 amino acid residues of FHF1B are essential for binding to rNa(v)1.9a. FHF1B did not interact with C termini of two other sodium channels hNa(v)1.7a (hNaNE) and rNa(v)1.8a (SNS), which share 50% similarity to the C-terminal polypeptide of rNa(v)1.9a. FHF1B is the first growth factor found to bind specifically to a sodium channel. Although the functional significance of this interaction is not clear, FHF1B may affect the rNa(v)1.9a channel directly or by recruiting other proteins to the channel complex. Alternatively, it is possible that rNa(v)1.9a may help deliver this factor to the cell membrane, where it exerts its function.