THE importance of interspecific competition for the distribution and abundance of organisms has been hotly debated during the last decade1-7. Although many field experiments have shown effects of interspecific competition on abundance and reproduction1,3, there is no unequivocal experimental evidence that interspecific competition can influence rates of local extinction in the field. Here I report that in a long-term field experiment with artificial rockpools, interspecific competition between three common rock-pool zooplankton species led to increased local extinction rates. In addition, studies of the distributional dynamics of the species in natural rockpools also showed that interspecific competition increases extinction rates. These results imply that interspecific competition is likely to limit the regional richness of species in the rockpool metapopulation system. MacArthur and Wilson8 were the first to suggest that an increase in extinction rate per species with an increase in the number of species could influence species richness on islands. Moulton and Pimm9 found that this was so among birds introduced to the Hawaiian islands, but the present study is the first field experiment providing unequivocal evidence of the effect.