It has been proposed that inbreeding contributes to the decline and eventual extinction of small and isolated populations(1,2), There is ample evidence of fitness reduction due to inbreeding (inbreeding depression) in captivity(3-7) and from a few experimental(8,9) and observational field studies(10,11), but no field studies on natural populations have been conducted to test the proposed effect on extinction, It has been argued that in natural populations the impact of inbreeding depression on population survival will be insignificant in comparison to that of demographic and environmental stochasticity(12,13). We have now studied the effect of inbreeding on local extinction in a large metapopulation(14) of the Glanville fritillary butterfly (Melitaea cinxia)(15). We found that extinction risk increased significantly with decreasing heterozygosity, an indication of inbreeding(6), even after accounting for the effects of the relevant ecological factors, Larval survival, adult longevity and egg-hatching rate were found to be adversely affected by inbreeding and appear to be the fitness components underlying the relationship between inbreeding and extinction. To our knowledge, this is the first demonstration of an effect of inbreeding on the extinction of natural populations. Our results are particularly relevant to the increasing number of species with small local populations due to habitat loss and fragmentation(16).