The similar to 25 N-terminal "HAfp" residues of the HA2 subunit of the influenza virus hemagglutinin protein are critical for fusion between the viral and endosomal membranes at low pH. Earlier studies of HAfp in detergent support (1) N-helix/turn/C-helix structure at pH 5 with open interhelical geometry and N-helix/turn/C-coil structure at pH 7; or (2) N-helix/turn/C-helix at both pHs with dosed interhelical geometry. These different structures led to very different models of HAfp membrane location and different models of catalysis of membrane fusion by HAfp. In, this study, the interhelical geometry of membrane-associated HAfp is-probed by solid-state NMR The data are well-fitted to a population mixture of dosed and semiclosed structures. The two structures have similar interhelical geometries and are planar with hydrophobic and hydrophilic faces. The different structures of HAfp in detergent vs membrane could be due to the differences in interaction with the curved micelle vs flat membrane with better geometric matching between the dosed and semiclosed structures and the membrane. The higher fusogenicity of longer sequences and low pH Is-correlated with hydrophobic surface area and consequent increased membrane perturbation.