To distinguish between the influences of area and isolation on the butterfly faunas of British islands two approaches are adopted. First, species richness is related to island area, isolation and the size of the faunal source. Neither area nor isolation account for much variance in species richness, though area is more important than isolation. In contrast, species richness corresponds closely to the size of the faunal source on nearby islands and to that at proximate locations on adjacent mainlands. The second approach relates the incidence of species on islands to their ecological attributes. A very close relationship is found between species incidence on islands and those ecological variables that measure potential for migration and colonization and that resist extinction. The implications are that the majority of British islands in this survey are insufficiently isolated to prevent intermittent migrations of butterflies to them or so small as to generate frequent extinctions. Independent data indicate the capacity of many resident species to migrate distances in excess of the isolation of most of the islands. Some evidence also exists for the long-term survival of species on islands; important considerations in this respect are that most islands in the survey are large compared to habitat patches sustaining species on mainland Britain and that substantial portions of islands are retained in early seral stages or comprise long-lived stable habitats (e.g. peat mosses) that are particularly suitable for many British species.