In the summer of 1973, about 5,000 pairs of adult Sooty Terns abandoned a specific part of their breeding grounds on Bird Island in the Seychelles. Incubated eggs and newly hatched chicks were left unattended and the area was not reoccupied in 1974. Numerous Ornithodoros (Alectorobius) capensis occurred in the deserted part in both years, but few or none in the area where bird breeding was normal. Persons visiting the deserted area were bitten by many ticks and experienced severe pruritus persisting for several days. In 1973 and 1974, Soldado (SOL) virus (Hughes serogroup) was isolated from the ticks taken on Bird Island from the ground and from sick and not visibly sick chicks of the Sooty Tern, and also from nests of the Blue-faced Booby on Des Noeufs Island. When ticks from the ground and from Sooty Tern chicks were fed on domestic chicks, they transmitted SOL virus and caused the death of their hosts. Soldado virus was previously known only from O. (A.) capensis-group ticks infesting marine bird colonies on islands off Trinidad and in a lake in Ethiopia. We have also isolated SOL virus from another species of the subgenus Alectorobius from Wales, Great Britain. Marine bird migrations undoubtedly account for the remarkably extensive distribution of SOL virus. The role of migration in the mechanisms and dynamics of tick and virus distribution, and the comparative growth cycles of Hughes serogroup viruses in Ornithodoros ticks, remain to be determined for a better epidemiological understanding of this agent.