In mainland France, virological surveillance of seasonal influenza is carried out by two National Reference Centers (CNR) for influenza viruses: one in the North region (Institut Pasteur, Paris) and one in the South region (Hospices Civils de Lyon, Bron), as well as by the Grog network (Regional Influenza Surveillance Groups). The objectives of this surveillance are to detect and isolate circulating influenza viruses early and to determine their antigenic characteristics. This virological surveillance system thus makes it possible to monitor the match between circulating influenza viruses and the vaccine composition, as well as to identify the potential emergence of new variants with epidemic or even pandemic potential. From the onset of the 2009 influenza A(H1N1) epidemic, the CNRs, assisted by regional P3-level biosafety laboratories, were mobilized to ensure continuous virological surveillance. In September 2009, virological surveillance based on samples from Grog physicians was also reactivated.