Résumé : The mode of orientation of gramicidin A in the β6.3 conformation in a monolayer of lysophosphatidylcholine was studied on the basis of conformational data. The approach involves localization of the centers of hydrophobicity and hydrophilicity of the gramicidin molecule, and takes into account the dielectric constant discontinuity of the interface. The most probable orientation is that with the C-terminal, tryptophan-rich, end directed towards the hydrophobic medium. In this orientation, four lysophosphatidylcholines surround a gramicidin monomer leading to a complex with an overall cylindrical shape compatible with the bilayer structure found in aqueous dispersions of mixtures of these molecules (Killian, J.A., De Kruijff, B., Van Echteld, C.J.A., Verkleij, A.J., Leunissen-Bijvelt, J. and De Gier, J. (1983) Biochim. Biophys. Acta 728, 141-144). © 1986.