During an extensive virological survey (1977-1986), 187 strains of five different arboviruses were isolated from 7,117 ticks parasitising seabirds in Brittany and Normandy, whereas only one viral strain was obtained from 1,414 ticks collected from the Southern coasts of France and Corsica. Among the many ecological factors involved in virus circulation in the two areas, the unequal sizes of the seabird colonies may probably explain the unbalanced distribution of viruses.