A DNA copy of the gene coding for the influenza A/Aichi/2/68 haemagglutinin protein was cloned in the plasmid pBR322 and the complete nucleotide sequence determined. Comparison of this primary structure and the deduced amino acid sequence with the haemagglutinin gene and protein of strains belonging to the same (H3) subtype and to different subtypes, of both human (H2) and avian (Hav1) origin, documents further at the molecular level the two independent modes of antigenic variation of the virus--drift and shift.