Studies with specific antisera to the haemagglutinin and neuraminidase antigens of all the influenza A subtypes show that A/turkey/Wisconsin/66 influenza virus, originally included in the Hav6 subtype, does not react in either haemagglutinin inhibition or immunodiffusion tests with antisera to Hav6. It is therefore proposed that A/turkey/Wisconsin/66 be placed in a new subtype designated Hav9. The neuraminidase antigens of the Hav6 subtype were further characterized and were shown to be N1, N2, Neq2, and Nav5 subtypes. Hav6 influenza viruses isolated from turkeys over an 11-year period showed little antigenic drift. The haemagglutinin and neuraminidase of A/shearwater/Australia/72 (Hav6Nav5) were identical with those of a virus isolated 8 years previously from a turkey in California: A/turkey/California/64 (Hav6Nav5).