The dissociation constants for H-3-saxitoxin to brain membranes and to skeletal muscle membranes of puffer fish Fugu pardalis have been estimated to be 190- and 460-fold, respectively, larger than those to corresponding membranes of rat, by a rapid filtration assay, while these values for H-3-PbTx-3 have been estimated to be one-third and one-half of those to rat, respectively. We have obtained a cDNA, encoding an entire voltage-gated Na+ channel alpha-subunit (fMNa1, 1880 residues) from skeletal muscle of F. pardalis by composition of the fragments obtained from cDNA library and RT-PCR products. In fMNa1 protein, the residues for ion-selective filter and voltage sensor and the charged residues in SS2 regions of domains I-IV were conserved, but the aromatic amino acid (Phe/Tyr), commonly located in the SS2 region of domain I of tetrodotoxin-sensitive Na+ channels, was replaced by Asn. With this particular criterion, we propose that the fMNa1 protein is a tetrodotoxin-resistant Na+ channel.