Substitution of a single Aib-residue in a peptaibol with (R)- and (S)-trifluoromethylalanine yields two local orientational constraints theta by solid state F-19 NMR. The structure of the membrane-perturbing antibiotic alamethicin in DMPC bilayers was analyzed in terms of two angles tau and rho from six such constraints, showing that the N-terminus (up to a kink at Pro14) is folded as an alpha-helix, tilted away from the membrane normal by 8 degrees, and assembled as an oligomer. The new F-19 NMR label CF3-Ala has thus been demonstrated to be highly sensitive, virtually unperturbing, and ideally suited to characterize peptaibols in membranes.