The specific assignment of resonances in the 300-MHz 1H nuclear magnetic resonance (NMR) spectrum of anthopleurin-A, a polypeptide cardiac stimulant from the sea anemone Anthopleura xanthogrammica, is described. Assignments have been made using two-dimensional NMR techniques, in particular the method of sequential assignments, where through-bond and through-space connectivities to the peptide backbone NH resonances are used to identify the spin systems of residues adjacent in the amino acid sequence. Complete assignments have been made of the resonances from 33 residues out of a total of 49, and partial assignments of a further 3. The resonances from several of the remaining residues have been identified but not yet specifically assigned. A complicating factor in making these assignments is the conformational heterogeneity exhibited by anthopleurin-A in solution. The resonances from a number of amino acid residues in the minor conformer have also been assigned. These assignments contribute towards identification of the origin of this heterogeneity, and permit some preliminary conclusions to be drawn regarding the secondary structure of the polypeptide.