A combination of the nuclear magnetic resonance (NMR) technique and the molecular modelling method has been successfully applied to determine high resolution solution conformations of a 17-residue oligopeptide LQARILAVERYLKDQQL (583-599) of gp41 from human immunodeficiency virus type I (HIV-1), and its mutant with Ala${ ightarrow}$Thr at position 589. Various two-dimensional NMR methods such as NOESY (two-dimensional NOE SpectroscopY), ROESY (two-dimensional Rotating-frame nOE SpectroscopY), TOCSY (TOtal shift Correlation SpectroscopY), and COSY (shift COrrelation SpectroscopY) have been used to obtain complete H-1 resonance assignments. Interproton distances obtained from NOE were input for a subsequent restrained molecular dynamics simulation. The overall shapes of both peptides are ${alpha}$-helical, except at the N- and C-termini. The threonyl side chain at position 589 in the mutant peptide protrudes outwards from the helical axis more than the alanyl side chain at the same position, which might account for differences in the antibody recognition patterns for the two peptides.