Nikkomycin X is a peptidyl-nucleoside antibiotic that inhibits chitin synthesis in fungi. Information on its conformation in aqueous solution was obtained from pH titration studies in which the nmr shifts of exchangeable and nonexchangeable protons were monitored. These studies and nuclear Overhauser effects support an unfolded or conformationally flexible structure for the antibiotic, and the syn and anti conformations of the nucleoside moiety were found to coexist. The conformation of the ribose ring was determined using a two-state model; a slight shift from type N to type S conformers occurred as the pH was raised from 1 to 6.