Peptide de7a was purified from the venom of Conus delessertii, a vermivorous cone snail collected in the Yucatan Channel, Mexico. Its amino acid sequence was determined by automatic Edman degradation after reduction and alkylation. The sequence shows six Cys residues arranged in the pattern that defines the O-superfamily of conotoxins, and several post-translationally modified residues. The determination of its molecular mass by means of laser desorption ionization time-of-flight mass spectrometry (average mass, 3170.0 Da) confirmed the chemical data and suggested amidation of the C-terminus. The primary structure (ACKOKNNLCAITgammaMAgammaCCSGFCLIYRCS*; O, hydroxyproline; gamma, gamma-carboxyglutamate; *, amidated C-terminus; calculated average mass, 3169.66 Da) of de7a contains a motif (gammaCCS) that has previously only been found in two other toxins, both from molluscivorous cone snails: TxVIIA from Conus textile and gamma-PnVIIA from Conus pennaceus. These toxins cause depolarization and increased firing of action potentials in molluscan neuronal systems, and toxin gamma-PnVIIA has been shown to act as an agonist of neuronal pacemaker cation currents. The similarities to toxins TxVIIA and gamma-PnVIIA suggest that peptide de7a might also affect voltage-gated nonspecific cation pacemaker channels.