The venoms of sea snakes (family: Hydrophiidae) contain potent neurotoxins which bind to the acetylcholine receptor in the neuromuscular junction. A major toxin was isolated from the venoms of the sea snakes Lapemis hardwickii and Enhydrina schistosa according to previously published methods. These pure toxins were studied by laser Raman spectroscopy to elucidate further the structure-function relationship to the neurotoxin. Spectra were obtained from the powder, aqueous solution, and deuterated derivatives of each toxin. The peptide backbone conformation of these neurotoxins was found to be of "anti-parallel beta configuration," as distinct amide I and III bands appeared at 1672 and 1240 cm-1, respectively. No indication of alpha helical structure in the neurotoxins was observed from amide I and III bands. This was further confirmed by the spectra of the neurotoxins after deuterium exchange. The peaks due to a single tyrosine residue appeared at 644, 834, and 846 cm-1. The intensity ratios of the toxin from L. hardwickii venom were 0.92, 1.0, and 0.84 at 644, 834, and 846 cm-1. It is concluded that the tyrosine residue is involved in some unusual intramolecular interactions and not readily accessible to water molecules. Similar results were obtained for the toxin of Enhydrina schistosa (common sea snake). The fact that only 50% of the tyrosine molecule is modified by nitration is in complete agreement with laser Raman result. The lack of a sharp Raman line at 1361 cm-1 suggested that the single tryptophan residue may be "exposed." The previous demonstration that the tryptophan residue can be modified readily with different reagents confirms these results. A relatively symmetrical disulfide peak at 512 cm-1 indicates that the geometry of the C-C-S-S-C-C linkage is nearly identical for all four disulfide bonds in the molecule. The absence of phenylalanine was established by the lack of a phenylalanine peak in the laser Raman spectra and by amino acid analysis.