HISTORICAL ecology contains various examples of how predators introduced onto islands by man have apparently exterminated native prey species(1-6). Conversely, a pioneering experiment(7) showed an increase in number of species with predator presence. Subsequent experiments have shown both increases and decreases in prey diversity(8-10). Here we investigate how predator introduction affects one aspect of prey diversity (number of species or species richness), and prey abundance, We ran a seven-year experiment on an entirely natural system of small islands, using the commonest local lizard as the predator and web spiders as prey. Lizard introduction caused rapid and devastating effects on spider diversity and abundance : within two years, islands onto which lizards had been introduced became almost identical to islands with natural lizard populations. The proportion of species becoming extinct was 12.6 times higher on ’lizard-introduction’ islands than on islands without lizards. Locally common and rare species were both reduced by the introduction of lizards, but nearly all of the latter became permanently extinct.