FMRFamide-gated Na channel (FaNaC) is a peptide-gated sodium channel in the epithelial Na channel/degenerin family. Although there are some data on the location of the putative peptide binding site, there is no structural information on the activation gating of FaNaC. Here, we addressed the function of a conserved aspartate residue in the second transmembrane domain of FaNaC. We used Aplysia kurodai FaNaC (AkFaNaC) and examined the function of the aspartate (D552) by site-directed mutagenesis and electrophysiological recording in Xenopus oocytes. We found that the macroscopic activation, desensitization, and potency of FMRFamide and its modification by external Ca2 and Mg2 are greatly affected by physicochemical properties of the amino acid at position 552. We conclude that D552 is situated in a key position that affects the gating properties of FaNaC.