Since 1999, several serotypes of bluetongue virus (btv) have been isolated in the western part of the Mediterranean basin, and since 2000, Corsica has been exposed to three different serotypes: BTV serotype 2 in 2000, BTV serotype 4 (BTV-4) in 2003 and BTV serotype 16 in 2004. In 2000 there were no surveillance systems for bluetongue, but in 2003, active surveillance of the circulation of BTV and its vector Culicoides species, aided by a raised level of awareness in farmers and veterinarians, made it possible to study the introduction of BTV-4. The monitoring and analysis of the seroconversions of sentinel herds of goats, clinical signs and meteorological variables showed that the serotype had been present in the island since May that year, but clinical signs were first observed only in October. Moreover, the weather conditions and wind patterns were suitable for the transport of Culicoides species from Sardinia in May. These observations suggest that btv had been transported on air currents from a southern infected area, and that it could have spread without causing clinical signs of disease for a few months.