High-resolution H nuclear magnetic resonance (NMR) spectroscopy at 300 MHz has been used to study the conformation in aqueous solution of the polypeptide toxin I (ATX I) from the sea anemone Anemonia sulcata. Resonances from a number of aromatic and methyl groups have been assigned, in many cases to specific amino acid residues in the sequence. Unusual splitting patterns due to virtual coupling are observed for the aromatic protons of one Trp residue and t-CH3 of one Thr residue. ATX I appears to be a flexible molecule but possesses a stable, nonrandom tertiary structure. The latter is characterized by a hydrophobic region encompassing Trp-23 and -31 and several aliphatic residues. A limited region of the molecule coexists in two equally populated structural forms, which are detected by splitting of the methyl resonancss from Met-18 and a Thr residue. The aromatic rings of Phe-25 and Tyr-42 are not involved in strong interactions with other residues. The pH and temperature dependencies of the spectrum have been analyzed. pKa values are obtained for Gly-1 (8.3), Lys-7 and -45 (11.2), Glu-35 (4.1), Tyr-42 (10.4), and one of the two remaining carbox-ylates, tentatively assigned to Asp-9. Protonation of the latter, at around pH 3, is accompanied by an extensive conformational change.