After treatment of different strains of influenza A at low pH, the threshold pH, at which the infectivity was lost, depended on the haemagglutinin (HA) subtype of the virus strain. Strains with noncleaved HA were much more stable when compared to strains with cleaved HA. These observations might explain why duck influenza viruses spread well by lake water, while highly pathogenic strains with cleaved HA do not. There were also significant differences in heat stability of infectivity among influenza A strains, which do not correlate with differences in stability at low pH.