The ability of human immunodeficiency virus type 1 (HIV-1) to fuse its membrane with the membrane of the target cell is a function of a approximately 23-amino-acid amino-terminal segment of the gp41 subunit of the envelope glycoprotein complex, known as the fusion peptide. The sequence of the fusion peptide is highly conserved among different variants of HIV-1 and is also very similar to that of HIV-2 and SIV. The fusion peptide is very hydrophobic and has a high content of glycine and alanine residues. Representation of the fusion peptide of HIV-1 as an alpha-helix predicts that most glycine residues would be found on one face of the alpha-helix. To assess the importance of the glycine residues for the fusogenic activity of the envelope glycoprotein complex, we mutagenized each glycine residue in the fusion peptide individually to a valine residue. The mutant envelope constructs were tested for their ability to induce syncytia (cell/cell fusion) and to mediate infection (virus/cell fusion) of CD4-positive cells. The results of our analyses show that two glycine residues (G10 and G13) located within the sequence FLGFLG in the middle of the fusion peptide are critical for syncytium formation and for the establishment of a productive infection, whereas other glycine residues (G3, G5, and G20) are more permissive to substitutions. Mutation of each of the two phenylalanines (F8 and F11) of the FLGFLG sequence to valine also decreased fusion, although to a lesser extent than mutation of G10 and G13. These observations demonstrate that G10 and G13 are critical elements of the fusion peptide and suggest that, in addition to hydrophobicity, the exact amino acid composition and structure of the fusion peptide are critical for function.