In response to the threat posed by the 2009 influenza A (H1N1) virus, an influenza surveillance system had to be rapidly established in Mayotte to detect the introduction of the virus and monitor its circulation and impact on the health of the local population. Surveillance relied on various sources of information, including a network of sentinel physicians, monitoring of hospital emergency department activity, virological surveillance, and tracking of severe cases. Surveillance identified an influenza epidemic lasting eight to nine weeks, peaking in weeks 37 to 40 and followed by a rapid decline. The implementation of a multi-source influenza surveillance system in Mayotte has contributed to the overall strengthening of infectious disease surveillance on the island. (R.A.)