The three-dimensional structure of the membrane glycoprotein neuraminidase of an escape mutant of the influenza virus strain A/Tokyo/3/67 has been determined to 3 A (1 A = 0.1 nm) resolution by X-ray diffraction. The mutant virus, selected by growing the virus in the presence of a monoclonal antibody to the neuraminidase, is shown to have undergone a single amino acid change of lysine to glutamic acid at residue 368. The three-dimensional structure of the neuraminidase is identical with that reported for A/Tokyo/3/67, except for a purely local adjustment of the structure at position 368.