The antimicrobial activity of amphipathic -helical peptides is usually attributed to the formation of pores in bacterial membranes, but direct structural information about such membrane-bound state is sparse. Solid state ²H-NMR has previously shown that the antimicrobial peptide PGLa undergoes a concentration dependent re-alignment from a surface-bound S-state to a tilted T-state. The corresponding change in helix tilt angle from 98° to 125° was interpreted as the formation of homodimers, residing on the bilayer surface. Under no conditions so far has an upright, membrane-inserted I-state been observed, in which a transmembrane helix alignment would be expected. Here, we demonstrate that PGLa is able to assume such I-state in a 1:1 mixture with magainin 2 at a peptide-to-lipid ratio as low as 1:100 in dimyristoylphosphatidylcholine/dimyristoylphosphatidylglycerol model membranes. This ²H-NMR analysis is based on seven orientational constraints from Ala-3,3,3-d₃, substituted non-perturbingly for four native Ala residues as well as two Ile and one Gly. The observed helix tilt of 158° is rationalized by the formation of heterodimers. This structural synergistic effect between the two related peptides from the skin of X. laevis correlates very well with their known functional synergistic mode of action. To our knowledge, this example of PGLa is the first case where an -helical antimicrobial peptide is directly shown to assume a transmembrane state that is compatible with the postulated toroidal wormhole pore structure.