Influenza surveillance by the National Influenza Virus Reference Centers (CNR) has addressed various objectives depending on the epidemiological context. Initially, the CNRs established appropriate diagnostic tools and mobilized their networks. Then, as the first signs of the epidemic wave emerged, they adapted their response and organizational structure to enable the most comprehensive analysis possible of the viruses and samples to be tested. Finally, it was also necessary to conduct real-time virological surveillance of adaptation factors, virulence factors, and the emergence of antiviral resistance. In total, during this epidemic wave—which mobilized tools and resources at a level far exceeding that of a typical seasonal epidemic—103,352 samples were analyzed by the "Influenza A" Laboratory Network, the National Laboratory Network (Renal), and the network of Regional Influenza Surveillance Groups (Grog). A total of 24,279 positive cases were detected between May 2009 and February 2010, with a weekly positivity rate ranging from 0% to 43%. The epidemic wave was observed from week 43 to week 52 of 2009, and the peak in detection of the A(H1N1)2009 virus was noted in week 48, with 3,877 documented cases. Phylogenetic analyses did not reveal the emergence of genetic variants; rare mutants with a possible virulence factor were detected in patients with severe forms of the disease. (R.A.)